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CHAPTER 3

DISC BRAKE SYSTEMS

Caution 1: The information in this chapter is based on general knowledge and may not be specific to your vehicle. Specific information is available in the factory service manual for your vehicle, which should be consulted for torque specifications, procedures, and safety concerns.

Caution 2: Some newer vehicles with electronic brake systems require a scan tool for servicing. See “New Technology Precautions” at the end of this chapter before servicing a vehicle with an electronic brake system.

Most modern vehicles use disc brakes on the front and disc or drum brakes on the rear. Because the front brakes do as much as 80–90 percent of the work when stopping, they tend to wear out quicker, requiring more frequent replacement than the rear brakes.

Drum brakes were used on the front and rear of cars into the 1970s. Since then, disc brakes have taken the place of drums on the front and in many cases on the rears too because they offer many advantages over drum brakes. They are much better equipped to dissipate the heat generated by modern vehicles, are easier to service, and are better suited for operation in wet conditions. They also self-adjust more reliably than drum brakes and resist brake fade (brake fade is the term used by engineers to explain a temporary loss of braking, usually due to heat). A drum can expand away from the brake shoes during heavy use, causing a lower brake pedal, but disc brake rotor expansion does not cause this condition when hot. Disc brakes also can shed gases and wear particles better than drum brakes and are less likely to cause the vehicle to pull to one side during stopping.

Today’s front-wheel-drive vehicles, SUVs, and light trucks have most of the weight up front, which is why engineers design brake systems to have more brake “bias” to the front end. If the same amount of brake force was present on the rear as the front during a hard stop, the rear brakes could lock up. This could lead to a loss of control, especially if the lockup occurs during a turn.

Disc brakes are common due to their advantages over drum brakes, but drum brakes are still found on the rear of some vehicles. Every year, more vehicles are produced with disc brakes on all four corners.

The pressure from this C-clamp is the same on both sides of the wood, even though the threaded shaft only presses on one side. One of Newton’s laws is that “For every action, there is an equal and opposite reaction.”

Operation

Imagine a large C-clamp around a 2x4 board. As you tighten the threaded shaft, both sides of the board are squeezed by the clamp the same amount, even though you are only tightening the screw against one side. Sir Isaac Newton stated: “For every action, there is an equal and opposite reaction.”

Calipers

The modern disc brake system is designed to clamp the brake pads against the spinning rotor to slow or stop the vehicle when you apply the brakes, much like the C-clamp. This device is known as a caliper. Hydraulic pressure from the master cylinder is developed when you press the brake pedal. This pressure moves the piston(s) in the caliper toward the pad(s) just like tightening the drive screw on the clamp.

On sliding or floating calipers, the piston(s) extends out of the caliper when the brake pedal is applied, causing one brake pad to make contact with the rotor (the action). As soon as contact is made, the caliper slides the other way (the reaction), which causes the other pad to make contact with the rotor.

This action and reaction clamps the pads onto the spinning rotor to slow or stop the vehicle. This design is called a sliding or floating caliper because that is exactly what it does. Sliding calipers ride on metal tracks, and floating calipers ride on pins or bolts with special seals and lubricants. It is easy to understand the concept if you visualize the C-clamp’s operation.

After you remove your foot from the brake pedal, a square-cut seal around the piston flexes back and acts as a return spring. It slightly retracts the piston so that the pads move away from the rotor just enough to allow free rotation. This ring sits in a groove in the bore around the piston; so, it is not actually mounted on the piston like an engine piston ring. The piston can be made of steel, aluminum, or a phenolic material. Phenolic is a synthetic material with superior heat resistance.

A caliper consists of a housing with a cylinder for hydraulic pressure to push the piston into the brake pad and into contact with the rotor. A seal in the bore of the cylinder allows the piston to build pressure and force the pads into the rotor. A dust seal (arrow) keeps out contaminants.

This C-clamp represents the way that the brake caliper squeezes the brake pads on both sides of the rotor equally to bring the vehicle to a stop. A piston takes the place of the threaded shaft, but the theory of operation is the same. As the piston pushes the pad into one side of the rotor, the caliper slides the other way and applies the other pad.

This caliper piston is made of steel but can also be made of aluminum or phenolic material. It rides in a seal in the cylinder bore. Because it has a larger diameter than the master cylinder piston, a hydraulic advantage exists, which means that a greater force is applied to the brake pads to help stop the vehicle.

The sealing ring (arrow) in a caliper piston is different than that of a ring on a piston in an engine. It is in a groove in the bore of the cylinder rather than on the piston itself. The groove it rides in is chamfered so that the seal flexes when the brakes are applied.

The sealing ring around the caliper piston is known as a square-cut seal. After the piston moves out to apply the pads during braking, it is retracted as the seal relaxes to its normal shape after being deflected. This reduces brake drag and improves fuel economy.

The brake caliper holds the pads in position and is bolted to the steering knuckle. A separate bracket may be used between the caliper and the steering knuckle on some applications. Stamped steel metal slides or guides and anti-vibration hardware or clips sit between the pads and caliper or bracket. This hardware often puts a small amount of spring tension on the pads to hold them in position to reduce vibration and prevent brake squeals, squeaks, and rattles.

Most disc brake systems use shims on the back of the pads, sheet metal guides, and spring clips to reduce brake pad rattle and squeal. Without these components, the pads would likely cause unwanted noise.

A brake pad contains a specific formulation of components to provide adequate stopping with a minimum of noise, dusting, and wear. Various formulations are used to handle the demands of heavy loads and heat or the extremes of emergency vehicles and race vehicles.

This single-piston floating caliper is a common design. It is lightweight and easy to service while still able to supply good stopping power and heat dissipation. A growing number of calipers are made of aluminum to reduce weight.

Some vehicles use a dual-piston floating caliper. The pistons are on one side of the rotor, so the caliper must still slide freely to engage the outboard pads during braking just as the single-piston caliper does. The dual-piston design can apply more force evenly across the pad for greater stopping power.

Most vehicles use a sliding or floating caliper design with either one or two pistons on one side of the rotor. It is imperative that the caliper slides freely for the brakes to work properly and the pads to wear evenly. The advantages to this design are that it is lightweight, easy to manufacture, low cost, and not prone to leakage. The disadvantages are that flexing can occur because the piston(s) is only on one side of the rotor. This can cause tapered pad wear. The slides can become contaminated and drag, leading to noise and one pad wearing more than the other.

A few vehicles use a fixed caliper, meaning that it does not slide. It has one or more pistons on each side of the pads that apply at the same time when the brake pedal is pressed. These calipers are usually heavier and have more potential for leaks, but they are known for excellent stopping and extremely even pad wear. Their weight dampens noise, and this generally results in minimal brake squeak or squeal. The greater weight is also a factor in vehicle handling. Because the brake caliper is “unsprung,” its weight has a greater negative effect on suspension operation versus “sprung” weight. This can be mitigated through the use of aluminum and more expensive alloys.

This is a fixed caliper, meaning that it is rigidly mounted and does not slide like a floating caliper. It has pistons on both sides of the rotor and a passage or transfer tube for fluid to flow between the two halves. Fixed calipers are generally found on older vehicles and performance cars.

Fixed calipers were popular on performance cars in the 1970s but are not as common today. They are found on older vehicles, some modern exotics, and high-performance racing applications. Some of the latest GMC and Chevrolet light trucks are equipped with fixed calipers.

Low-Drag Calipers

Because of government Corporate Average Fuel Economy (CAFE) and emission control standards, low-drag calipers were designed some years ago. Cars of the 1970s with disc brakes did not have calipers that retracted much, so they dragged the pads on the rotors until flex in the components caused enough “knock back” to free the rotors. If you jacked up a car in those days and attempted to spin the wheel, you probably had to pry the caliper piston back a bit to allow the wheel to spin. This wasn’t considered to be a problem, however, until fuel prices increased in 1973. Engineers knew that a wheel that was easier to spin improved fuel economy, so low-drag calipers entered the scene.

A special square-cut seal around the caliper piston in the bore flexes when the brakes are applied. When the brakes are released, the seal flexes back, which pulls the piston back away from the pads like a spring. This results in free-wheel rotation much quicker and, as a result, better fuel economy. It also likely improves pad life due to less dragging on the rotors. When you use less fuel, the vehicle has lower emissions.

Quick Take-up Master Cylinders

Some manufacturers developed master cylinders that supplied more fluid volume when the brake pedal was first applied to counteract the extra piston retraction from the low-drag caliper design. Without the redesign, the pedal required more movement before the brakes applied. These master cylinders are known as “quick take-up” or “fast fill” master cylinders and usually have a larger bore feeding the primary chamber to get fluid to those wheels quicker and maintain an acceptable pedal height.

Parking Brake for Rear Discs

If disc brakes are used on the rear of a vehicle, they operate just like the front brakes but they are likely smaller and have the parking brake system built into them. Some applications use a corkscrew-like shaft that is actuated by a cable or electric motor drive to apply the parking brake. Others may have a small drum system inside the rotor for parking brake application, which is more effective due to self-energizing brake shoe action.

This rear caliper uses an integral parking brake that moves the piston into the pad when the cable is applied. It uses a corkscrew design inside the piston that is prone to rust and binding. The piston must be rotated as it is pressed back into the caliper with a special tool prior to pad replacement.

Some rear disc parking brakes use a small set of brake shoes inside of a drum cast inside the rotor. This is referred to as the rotor hat section. These shoes should never wear out because the rotor should be stationary when the parking brake is applied.

Brake Pads

Brake pads are relatively small pieces of friction material attached to a steel backing plate that press against the rotor under hydraulic force. A process known as sintering is used to make brake pads. Most pads are composed of several elements compressed together and heated in a mold.

The composition of the formula used varies from vehicle to vehicle and among different manufacturers. It can possess high percentages of different compounds such as organic material, semimetallic particles, ceramic, carbon metallic, Kevlar, or silicone carbide. Some pads excel in heat control, braking ability, and even reduce wheel dusting. Some wear faster than others but may offer more aggressive braking for racing or heavy loads. Other components include binders (glue) and fillers. Sometimes even ground-up walnut shells are used in the mix as a filler!

Brake engineers design tailored systems for the needs of each specific vehicle platform, often with a special patented mix of ingredients in the brake pad formula. Vehicle engineers must meet strict Department of Transportation (DOT) standards for stopping distance, and they often stamp the heat-handling ability onto to the edge of the pad or shoe. This code gives an indication of how much heat the friction material can handle when both hot and cold. This is called an edge code and is classified using letters of the alphabet, with higher letters meaning better heat handling (GG could handle more heat than EE, for example).

Most pads today are bonded, meaning that the friction surface is attached to a steel backing plate using high pressure and heat in a mold during manufacturing. Pad backing plates may have a rough surface with fish-hook-like teeth where friction is bonded to help the pad material remain on the plate. Older designs used rivets to attach the friction “puck” to the backing plate. Some newer designs have holes in the plate; friction material is pressed into it during manufacturing to help hold it to the plate during operation. Some even have a thin layer of friction material on the backside of the pad to act as a shim for noise control.

Some older disc brake designs used a brake pad with the friction material riveted to the steel backing plate. This type of pad cannot wear as much as a bonded pad without the rivets causing damage to the brake rotor. Some pad manufacturers placed a thin insulator between the friction material and backing plate for quieter operation.

Most brake pads in use today have the friction material bonded to the steel backing plate. During manufacturing, friction material is placed under high pressure and heat to form it and bond it to the backing plate.

Some manufacturers have holes through the steel backing plate so that friction material can flow through during manufacturing. This helps keep the friction material in place. One well-known manufacturer has a thin friction surface on the other side that acts as a very effective shim for noise reduction.

Pad Materials and Applications

A pickup truck designed to haul a 36-foot fifth-wheel camper that weighs 12,000 pounds would likely be equipped with semimetallic brake pads from the factory. This composition includes a high level of metallic particles in the formulation that are extremely good at drawing heat away from the rotors and conducting it into the suspension system and wheels.

For severe-duty applications such as police, ambulance, or racing, a pad containing even more metallic particles is available, often called severe-duty or carbon-metallic. These pads work well on extremely hot surfaces but do not grip well on cold rotors, so this formulation is not a good choice for a small lightweight sedan. Severe-duty pads also tend to be noisier and cause more extreme rotor wear due to their high metal content and aggressive nature.

German vehicles often use silicone-carbide formulations, which are also aggressive and can cause rotor wear and high wheel dusting, but the benefit is excellent stopping power. It is not uncommon for some German vehicles to require new rotors during every brake job because the aggressive friction wears the rotors.

Lighter weight vehicle applications used organic and asbestos formulations years ago. Asbestos was found to cause cancer, so it has been replaced with non-asbestos formulations. Some light-duty pads also contain copper particles. Brake pads containing copper have come under scrutiny because of the copper dust potentially being washed into drains on the street and entering the water supply.

Ceramic formulations are extremely popular on lighter weight applications. They are generally quieter and shed a lighter color dust, which improves wheel appearance. However, they tend to be less able to conduct heat away from the rotor. They act more like a barrier or insulator. This can cause the rotor to run hotter, so they are not a good choice for the truck pulling the camper mentioned earlier, even though less dust may be seen on that vehicle’s wheels. Some pads deposit a darker color dust and wear away some metal from the rotor for them to work properly. The bottom line is that it is not wise to sacrifice braking ability when pulling a heavy load to have cleaner wheels!

Brake pad manufacturers often use chamfered edges and slots in the friction surface to reduce vibration and wear particle accumulation. Both can cause brake noise.

There are no standards for the percentage of compounds required to call a pad a certain name, such as Kevlar or ceramic. Advertisers often use these as a way to help their products sell, even though the percentage may be extremely low.

There is no perfect pad for all applications just as there is no tire that can do everything for every demand. If a tire can last 80,000 miles, it will not be able to give good traction in rain and snow. If it is aggressive for off-road use, it will probably be extremely noisy on the highway. If it has amazing grip around turns, it probably won’t last long. Nearly everything on a vehicle is a compromise. That also applies to fuel efficiency versus speed. Fast sports cars typically use a lot of fuel. Little fuel-efficient cars aren’t fast.

In the same way, each brake pad has its own strengths and weaknesses, and the choice of pad material should not be based solely on price or how clean the wheels remain. On the internet, you can find pads for the same vehicle from $15 to $115, so it might be confusing to determine which pad is right for your application. Most quality pad manufacturers have catalog listings that show the factory formulation in bold print and options such as severe-duty or ceramic in lighter print. For the average driver, it is probably best to stay as close to the factory formulation as possible because the original equipment manufacturer (OEM) engineers likely spent a lot of time and money to decide that they were the best! Pads are sold in axle sets, so one box does both the left and right sides.

Some pads have chamfered edges and slots that are intended to reduce noise from vibration or to channel away gases and wear particles. Replacement pads may differ in appearance from originals. If they are from a trusted manufacturer, they might perform better than the OEM design, but be sure they fit properly and are listed for your vehicle in their catalog.

Even though the original OEM is held to strict government Federal Motor Vehicle Safety Standards (FMVSS 135) regarding original equipment brake performance, the aftermarket does not have many restrictions on brake pad material or design. It is best to stay with a name brand and not to shop for brake pads based on price alone.

Rotors

Most brake rotors are a one-piece design made of cast iron or a composite design using a steel plate with a cast-iron friction surface. Performance applications, such as late-model Corvettes, use carbon-ceramic rotors, which are not metal. They are expensive but handle heat extremely well and weigh about 11 pounds less than a comparable cast-iron rotor. This reduces unsprung weight, improving handling and ride and reducing overall vehicle weight, which improves performance and reduces emissions.

Many performance vehicles come from the factory with carbon-ceramic rotors. The main advantages are superior braking and weight reduction for handling, performance, and fuel economy. Less unsprung weight means better handling. Less overall weight (approximately 50 pounds) means improved performance, better fuel mileage, and lower emissions.

Often the front rotors are a vented design with cooling fins between the two friction surfaces. If your vehicle is equipped with rear disc brakes, the rotor can also be a one-piece plate without cooling fins because they don’t have to deal with the heat that the front brakes do.

Some performance vehicles are equipped with drilled and/or slotted rotors from the factory. This is to assist with channeling gases and wear material away from the contact surfaces between the rotor and pad and improve brake performance.

Aftermarket components to add these features to your existing vehicle are available. Purchase from a reputable source to be sure that you are getting an actual improvement in brake performance. Performance systems may include larger diameter rotors and upgraded calipers and can be quite expensive.

Use Caution with Brake Fluid

Some technicians prefer to wear disposable gloves when working around brakes and brake fluid.

You may not have the same “feel” at your fingertips when handling small parts such as copper washers, but disposable gloves are relatively thin so that they can work. They are usually available in boxes of 100 for less than $10. Caution: Brake fluid is extremely damaging to paint, so be extremely careful when you are working with it. ■

Vehicles may be equipped with drilled and slotted rotors to reduce heat and for channeling wear particles away from the rotor surface. Aftermarket brake upgrade kits with these features are also available for many applications.

Some may look good through the wheel but actually cause the brakes to run hotter due to a reduction in rotor mass. They could just be the factory type of rotor with holes drilled in them.

Inspection and Diagnosis

Disc brakes can become worn without displaying warning signs. There will be no change in pedal height or feel and the vehicle continues to stop until the friction surface is completely worn away. Some vehicles have audible wear sensors attached to the brake pads that make contact with the rotor surface, causing a high-pitched squeal when the pads are almost worn out. This is usually heard when the brakes are not applied.

It is sometimes difficult to view the condition of the pads and rotors without removing the wheel. This is especially true of the backside of the rotor and inboard pads. Some applications have a full backing plate on the inboard side of the rotor to protect it from contamination, which greatly restricts visibility.

You may be able to get a partial view of the outboard pad and rotor surface through the wheel, but a thorough inspection requires wheel removal to get a good look at the entire system because a backing plate hides the inboard pad and rotor surface from view.

Most brake pads have a metal wear sensor (arrow) that is designed to scrape against the rotor when the pads are almost worn out. Its purpose is to alert the driver that brake service may be required soon.

If the sensor is ignored, the metal backing plate or rivets make contact with the rotor face when the friction material wears away and causes severe damage to the rotor surface. If the metal-to-metal condition continues, the area where the pads contact the rotor can become completely worn away and the caliper piston can come out of the rotor, causing hydraulic failure and complete loss of braking on that circuit.

Some high-end vehicles have electronic sensors that cause a warning lamp on the instrument panel to appear when the pads are nearing the end of their life. This design generally uses a brush-like material to ground a circuit through the rotor.

Ignoring the warning signs can lead to more expensive repairs. This rotor was severely damaged because the friction material on the pad was completely worn away and metal-to-metal contact was the result. Much noise resulted but was apparently ignored.

This is an example of a brake pad that was worn past the friction surface and into the steel backing plate. Most of the time, the rotor is damaged to the point of requiring replacement.

Most luxury and German vehicles have electronic wear sensors. This type of sensor completes a circuit to ground when the pads are nearing the end of their life. A warning lamp on the instrument panel is illuminated so that the owner knows that brake service will be required soon.

Another telltale sign of brake pad wear is low brake fluid level in the master cylinder reservoir. If the fluid level is extremely low, a brake indicator lamp may illuminate on the instrument panel.

Low fluid level in the master cylinder is caused by a hydraulic system leak or disc brake pad wear. As the pads wear and their thickness diminishes, fluid must take up the void behind the caliper pistons, which leads to less fluid in the master cylinder. This master cylinder has a sensor (arrow) that turns on a dash light when the fluid level is low.

The Test Drive

The first step in assessing a disc brake system is a thorough test drive. Listen for noises and watch for pulling to the left or right when the brakes are applied. Observe the brake pedal height and feel of the pedal when stopping. See if the force required to stop the vehicle is abnormal. If the pedal moves up and down (pulsates), the rotors may need attention. If the pedal is unusually low, the drum brake system may need adjustment or repair. There could also be an issue with the ABS system.

On disc brakes, a low pedal does not result from worn brake pads because they are self-adjusting as fluid takes up the space behind the piston as the pads wear. A pull to one side during braking could be the result of a restricted brake hose, caliper, or pad issue or even a loose or worn wheel bearing. It could even be caused by a steering or suspension problem.

If the pedal is hard and the vehicle is difficult to stop, you may have a power booster failure or low engine vacuum.

If the pedal is spongy or soft, you may have air in the fluid. Some vehicles have hydroboost brake assist that receives fluid from the power steering pump to assist in stopping. Be sure the power steering fluid level is correct and the hoses are intact.

Visual Inspection

After the test drive, open the hood and wipe away any dirt that may have accumulated around the lid of the master cylinder. Remove the lid and observe the level of the fluid. You may be able to see through the reservoir on some applications without removing the lid. The fluid should fall between the minimum and maximum markings. If the fluid is low, it could be an indication of a leak or the brake pads could be worn.

Don’t let any fluid from the lid or your hands come in contact with any painted surface. Brake fluid damages painted surfaces. If you do accidently get brake fluid on a painted surface, flush the surface immediately with plenty of water.

Now it’s time to remove the front wheels to get a good look at the disc brakes. You may be able to see the outboard surface of the rotor and the outboard pad through the wheel, but to perform a thorough inspection, the wheel must be removed. Often a backing plate is used, which can limit visual inspection of the backside of the rotor surface and the inboard pads. Some pickups and SUVs that are not often used develop a layer of rust on the backside of the rotor, which greatly affects braking ability. This can easily be missed if the wheels are not removed during inspection.

Raising the Vehicle

To check the front brakes, you first chock both rear wheels, making sure the vehicle is in park or in gear with the parking brake applied and the engine off. If you’re using hand tools, remove the front wheel covers or center caps (if used) and loosen the wheel fasteners slightly. Carefully raise the vehicle using an appropriate method such as a floor jack placed as specified by your vehicle’s manufacturer.

When the vehicle is high enough to allow the front wheels to be removed, place jack stands under the vehicle and lower the floor jack to allow the vehicle weight to rest on the stands.

Be sure that you raise the vehicle per your vehicle manufacturer’s guidelines. The American Lift Institute (ALI) issues a guide that lists all vehicles and their lift points. Lifting information for your specific vehicle might also be available online.

A floor jack with adequate capacity should be placed at the proper lifting point for your vehicle and carefully lifted high enough to allow wheel removal. Do not use the jack that came with your vehicle to support it while servicing your brake system.

Wheel chocks should be placed on both sides of the opposing wheel when jacking up a vehicle. Also be sure that the vehicle is in park or in gear and the parking brake is applied before raising the vehicle. If you are using hand tools to remove the wheel fasteners, slightly loosen them before raising the wheel off the ground.

A jack stand with adequate capacity must be used to support the weight of the vehicle prior to beginning brake inspection or service. If the jack stand is placed under the vehicle as a safety backup and the jack is supporting the weight, it could be crushed if the jack should fail. This is referred to as “shock loading.” Jack stands are not rated for this type of load.

Remove the wheel fasteners and lift the wheel from the hub. Wheels are heavy, so be careful. Some manufacturers recommend marking the stud position and reinstalling the wheel in the same position.

Remove the wheel fasteners. Most vehicles use nuts that require one of the following socket sizes: 3/4, 13/16, or 7/8 inch or 17, 19, 21, or 22 mm. German vehicles and Saturn vehicles use wheel bolts instead of nuts, and most require a 17-mm socket for service. Some trucks and SUVs use larger lug nuts that require a larger socket. Some older vehicles have left-handed threads on the driver-side wheel studs.

If you are using hand tools, you may need to have an assistant hold down the brake pedal as you remove the fasteners to keep the wheel from spinning. Do not attempt to work on a vehicle if it is lifted unsafely. Don’t use the jack that came with your vehicle or wood blocks to support the vehicle during brake service. Only an automotive jack stand rated for the load should be used.

After the two front wheels are removed, you have a much better view of the disc brake system.

Brake Pad Inspection

The thickness of the friction material on most new brake pads is 1/4 to 1/2 inch. Every manufacturer has replacement guidelines for thickness and some states have vehicle inspections to ensure that the thickness is safe for vehicle operation. Some allow as little as 1/16 inch on bonded pads or 1/8 inch on riveted pads before the vehicle is rejected. Special tools are available to measure pad thickness on the vehicle using color coding. Some are able to obtain the actual thickness remaining without removing the pad first.

Several tools are available to measure brake pad thickness while the pads are still on the vehicle. The color-coded gauges indicate a range of thicknesses. The other tool has a scale to read exact dimensions.

To measure pad thickness, the tips of a scaled tool are placed between the rotor and the pad backing plate. The scale displays the remaining pad thickness in increments of 1/32 inch.

The color-coded “go-no-go” gauge is placed between the rotor surface and the pad backing plate like a feeler gauge. The color codes indicate whether the remaining pad thickness represents an immediate concern or not. The green gauges are the thickest and the red ones are the thinnest.

Here a color-coded gauge is in use. The yellow gauge slides in and out easily with light drag, indicating that the pad still has some life left.

Look for cracks that may affect operation or contamination from brake fluid or grease. If found, the pads should be replaced, not just cleaned. Grease can be absorbed into the pad structure. Pads can also be overheated. Generally you see evidence of this on the rotor surface in the form of blue, black, or brown heat marks or hot spots. If this found, rotor replacement is required.

Excessive uneven (wedge shaped) wear or one pad wearing more than the other could mean caliper slide issues. If the vehicle has a sliding or floating design and the outboard pad is worn more than the inboard, the slides are dirty. If the inboard pad is worn more than the outboard, the slides are seized. If you do not address these issues, the new pads will develop the same wear patterns.

Brake Noise

Unfortunately brake noise occurs from time to time. It is the most common reason that professional brake shops have vehicle “comebacks.”

Doing the job right and taking the proper steps during service minimizes the possibility of brake noise, but because all brakes vibrate and vibration can be audible, noises can happen. The goal of an automotive brake engineer is to make the frequency of the vibration outside of the range of human hearing.

Unusual weather dry spells can contribute to brake noise. Be sure to use new hardware installed correctly and proper lubricant in all the right places. A coarse rotor surface creates “mountain peaks” where the new pads ride. This reduces surface contact, which causes higher pedal effort, excess wheel dusting, and noise until the surfaces smooth out and comply with each other. The smoothness of the rotor face is measured in roughness average (RA); the lower the number, the smoother the face.

Aggressive braking after a brake job can cause overheating of the friction material, which brings the binders (glues) to the surface, known as glazing. This harder surface is prone to noise and can cause higher brake pedal effort until it wears away or is sanded to a flat surface. In many cases, the pads must be replaced when this happens.

Expected Brake Pad Life

It is difficult to predict just how long brake pads will last in terms of mileage or time because of varying driving habits and conditions. If a vehicle is used for towing or in constant stop-and-go traffic, the brakes generally require more frequent service. Younger drivers may go through pads more frequently due to last-minute stops. Some vehicles may require front brake service as much as three times more than rear brake service due to weight transfer and system design.

You may notice more brake dust on the front wheels than the rears when washing your vehicle. This is especially true on some European vehicles because many use more aggressive silicone-carbide brake pads resulting in more rotor wear and dark brown or black dust on the wheels. Some owners wax their wheels to minimize the dust formation.

If the front pads are severely worn or you find that they wear out frequently, be sure the rear brakes are working properly. Make sure the brakes aren’t dragging due to a misadjusted pushrod in the master cylinder, too much fluid, or a misadjusted brake light switch at the brake pedal. Brake hoses with an internal restriction can cause the brakes to stay on, causing abnormal wear.

In rare cases, an ABS issue can contribute to excessive pad wear. For example, a mechanical problem within the ABS modulator on some vehicles can eliminate rear brake function and cause accelerated front pad wear.

Rotor Inspection

Be sure to examine the rotor surfaces for deep grooves and excessive wear. Some rotors have internal cooling fins and some are solid. Most front rotors have fins because they run hotter than the rear brakes. If your vehicle has a brake pulsation when stopping, you may have excessive rotor thickness variation (also called parallelism), which can develop due to lateral rotor runout. Machining or replacing the rotors is required to fix this issue.

You may also need to replace the rotors if they are badly worn or the thickness is below the minimum specification. A micrometer and dial indicator are required to properly measure your rotors for overall thickness and variation. The typical amount of runout allowed by many manufacturers is 0.003 inch, and thickness variation standards are even tighter at 0.0005 inch. As you can see, tolerances are pretty critical for proper brake operation.

Your local auto parts store should be able to measure the thickness and compare it to OEM specifications found in the vehicle service manual to make a recommendation as to whether the rotor can be machined or must be replaced. The service information often lists the nominal thickness, which is the thickness of the rotor when it was new. It also may list a “machine to” thickness. This is the minimum thickness that the rotor can be after it is turned on a lathe. If the rotor has deep grooves, measurements should be made at the deepest point.

If the pads are worn unevenly it could be an indication that the caliper or bracket needs attention. The slides could be binding or a pin or slider bolt could be seized. Further disassembly should reveal the culprit. A slight amount of normal uneven wear does occur on some systems due to caliper flexing.

A micrometer or dial caliper can measure the remaining rotor thickness to determine if it is still within the manufacturer’s safety tolerance. A micrometer can also be used to measure the thickness at several locations to compare them and determine thickness variation (also known as rotor parallelism).

If service information only lists a minimum thickness, the rotor is generally machined to about 0.030 inch larger so that it is still within tolerance after it “wears in.”

You may also find specifications cast into the rotor or listed online. Be sure not to purchase poor-quality rotors if yours need replacement. Some shed iron particles that can embed in the new pads and cause noise. Impurities in the cast iron and improper heat treating during manufacturing are the common reasons for this. You don’t want to have to do the job again in a couple of months.

If the rotors are within specification and you have no pedal pulsation or deep grooves, some manufacturers approve simple pad replacement only. This “pad slap” practice is often debated among professionals. Some shops have no problem with this approach while others machine or replace rotors on every brake job. One reason for this is that most shops do not possess a micrometer that can measure down to 0.0005 inch to determine if the thickness variation is within specification. They suggest replacement or machining to ensure that it is corrected if it is out of specification. Other shops feel that if brake pulsation does not exist when test driving, a pad slap is an option.

Another concern is that every time a rotor is machined, it has slightly less mass and therefore could run hotter during braking. Some professionals are concerned about the quality of the replacement and suggest that the machined OEM rotor with less mass is still better than some brand-new inferior replacements.

You also want to inspect the rotor surface for evidence of overheating and cracks. Bluing and hot spots can develop and are reasons to replace rather than machine.

A dial indicator can be used to determine the amount of rotor lateral runout. It is attached to the suspension system or caliper bracket and a dial gauge placed on the rotor surface. As the rotor is rotated slowly, runout causes the needle to deflect.

Rotor thickness specifications are available in the vehicle service manual and are also cast into the rotor in most cases. This specification is generally the minimum thickness, not the amount the rotor can be machined down to.

New or Rebuilt?

There seems to be the trend toward a throw-away world. Sometimes this is because the replacement part may cost less than repairing the old part. It is common with engines, starters, alternators, and even transmissions. Years ago, brake calipers, wheel cylinders, and even master cylinders were commonly rebuilt at a workshop. Most of these parts are now replaced with new when they fail due to the labor required and limited availability of parts.

Many shops replace rotors and drums rather than machine them. The choice is not always an easy one when faced with a grooved or pulsating rotor. Some shops feel that some replacements from overseas may be of lesser quality and choose to machine the originals, but others find that the price is close enough to the machining cost and have had success with replacements, so they go that route.

High-quality rotors and drums are available in the industry from name-brand manufacturers, but it is not always easy to tell if you are buying a quality piece. Just remember that as in many things, you get what you pay for, and doing the job over because a part failed might cost you much more in the long run. Quality components are especially important if your vehicle is used to haul heavy loads or pull a trailer. ■

On-Car Lathes

Lathes for machining rotors on the vehicle have been around for quite a while, but they are becoming more popular. Most shops have a lathe that can turn both rotors and drums, but the drums must be removed and taken to the lathe for machining. This means that the hub that the rotor or drum mounts to can still have some runout and cause the rotor or drum to have runout even though it may have been machined perfectly on the lathe.

That’s where an on-car lathe is superior. It turns the rotor while mounted to the hub so that both assemblies are trued when the rotor is machined. The on-car lathe cannot machine drums, so they must either be removed for machining or replaced with new.

Caliper Inspection

One of the main areas to inspect the calipers are the slides or pins that the caliper must move on as the brakes are applied and released. As your vehicle ages, rust and corrosion can develop between the moving parts, creating issues. Calipers must be removed, surfaces properly lubricated, and parts replaced as necessary.

Binding slides cause uneven pad wear, possible noise, and inadequate braking. The pistons must be able to be pressed back into their bores if the calipers aren’t going to be replaced to allow room for the installation of the new brake pads. The pistons are extended out of their bores because of pad wear; fluid had to take up that space behind the piston. This is also why the fluid level in the master cylinder drops as the pads wear.

Another reason that the fluid level drops is a leak in the system, but that would likely be accompanied by a sinking pedal, fluid on the ground, and a red brake warning light on the dash. Most of the time, the master cylinder is completely empty if a leak were present.

Service

Caution: Some newer vehicles with electronic brake systems require a scan tool for servicing. (See “New Technology Precautions” at the end of this chapter before servicing a vehicle with an electronic brake system.)

Be sure to observe all safety precautions when performing brake service, including the use of wheel chocks, proper jack use and jack stands, safety glasses, ear protection, and disposable gloves. It is also wise to have good ventilation in the area and a fire extinguisher nearby.

Brake Fluid Condition

Brake fluid degrades over time. As fluid ages, it loses some of its additives. Although brake test strips are available to see if your fluid needs to be replaced, it’s a good idea to replace it during brake servicing.

Although some manufacturers don’t specifically recommend a replacement interval other than during hydraulic repair, it is a good idea to begin the job by using a turkey baster to remove all of the old brake fluid from the master cylinder. Immediately fill the master cylinder to the maximum mark with the correct fluid and install the lid.

Brake fluid test strips are available to determine the condition of the fluid. They detect the level of copper present to determine fluid age and additive degradation. If the fluid is in need of replacement, the test strip turns purple within a minute of being immersed in the fluid.

Common Hand Tools Come in Handy

Most tools needed for disc brake repair can be found in the average do-it-yourselfer’s toolbox. Bolts are generally metric in the range of 10 to 21 mm. Some vehicles use Torx or Allen-head fasteners. A torque wrench is needed to be sure all fasteners are safely tightened.

Tools for pushing back calipers are extremely handy but a C-clamp can be used against one of the old brake pads in most cases on the front. To push back rear caliper pistons that use integral parking brakes, a special tool that rotates the piston as it goes in is extremely useful.

If you are doing drum brake work, you need some inexpensive tools to remove and install hold-down and return springs. Tubing wrenches are the best for removing hydraulic fittings. A floor jack, jack stands, and wheel chocks are required for safe hoisting. ■

A color chart is supplied with the test strips. After the strip is immersed in fluid from the master cylinder, it begins to change color if the fluid contains high levels of copper. If it becomes dark purple, a fluid change is needed.

Most vehicles require DOT 3 brake fluid, but you should always check the vehicle manufacturer’s specification. Put the cap back on the brake fluid bottle immediately because an open container absorbs moisture from the atmosphere and the fluid is ruined in a short time. It is also wise to buy small bottles of fluid for this reason.

Remember that brake fluid takes paint off quickly, so be careful where you place your hands after handling brake fluid and don’t let any fluid drip on a painted surface. If it does, flush the surface with water immediately!

Brake Bleeder Removal

After you have raised and supported the vehicle, make sure the brake bleeders can be opened. This is necessary because the piston(s) in the caliper must be pressed back into its bore to allow room for the new, thicker brake pads. Also, the bleeder should be opened to allow potentially dirty fluid to escape instead of traveling upstream. If a bleeder is stuck, lightly tap straight onto it with a ball-peen hammer and use a six-point wrench or socket. Tighten it a bit first and then loosen it. Tapping around the bleeder on the caliper can also help to loosen it. Do not use heat, which can damage internal rubber components.

Test Strips Determine Amount of Copper

It is difficult to tell if brake fluid is bad by looking at the color of it. It naturally absorbs dye from the rubber components and turns darker as it ages, and most manufacturers don’t have service intervals listed in the manual. A pretty common guideline is to replace the fluid every time the system is opened for service. A handful of manufacturers have a time interval, such as 24 months, but most do not specify a time or mileage interval.

Until recently, most professionals used a battery-powered device to determine the boiling point of the fluid, but now most are using test strips in the master cylinder reservoir. These strips determine the copper content of the fluid. The older the fluid, the more copper it has absorbed from the steel brake lines. This is an indication of age and additive degradation. The Society of Automotive Engineers (SAE) has determined that brake fluid with 200 ppm (parts per million) of copper or more should be replaced. ■

Vacuum Bleeder Option

It is a good idea to first remove all of the old brake fluid from the master cylinder reservoir and pour in new fluid. This step keeps old fluid from entering the master cylinder and ABS unit when the system is opened up downstream. Brake vacuum bleeders are available for this purpose, but a common turkey baster works great if you have a container under the hood with you to squirt the old fluid into. Once the old fluid is out, you can pour in the correct fluid to the maximum line. The reservoir cap has the recommended fluid listed on it. ■

Prior to beginning the brake job, it is a good idea to use a vacuum bleeder or turkey baster to remove the old fluid from the master cylinder reservoir. This prevents old fluid from flowing through the master cylinder and into the ABS modulator.

After the old fluid is removed, pour in new brake fluid until it is between the minimum and maximum mark on the master cylinder reservoir. When the calipers are removed or the bleeder is opened, new fluid circulates through the master cylinder and ABS modulator instead of old fluid.

If the old calipers are to be reused, the bleeders must be checked to see if they open before starting the job. If they are unable to be opened without breakage, replacement calipers may be required. It is a good idea to open the bleeders prior to pushing back the caliper piston so that the old fluid can escape instead of flowing upstream through the ABS modulator and master cylinder.

If the bleeder is tight, try tightening it a bit first and then tapping it gently with a small hammer to free the rust around the threads. Do not use heat, which can cause internal damage to the caliper seals