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CHAPTER 2

BRAKE SYSTEM COMPONENTS

The modern automotive and light truck brake system is made of many components working together to achieve effective control while slowing or bringing the vehicle to a safe stop.

When you press the brake pedal in a vehicle with manual brakes, linkage connects the mechanical force to hydraulic pistons, which pressurize sealed systems of lines, hoses, and output pistons with high-pressure fluid that applies the friction material to the spinning rotor or drum to stop the vehicle. Think of a cylinder type of squirt gun. Pressing on the plunger causes water to squirt from the other end. If this end were connected by a hose to another cylinder full of water the same size, the movement of the input plunger would cause equal movement at the output plunger. This movement is used to apply the brake pads or shoes to stop the vehicle. If the output piston diameter were twice that of the input, the output would apply twice the force, but it would move half the distance.

On a vehicle, the cylinder applied by your foot is known as the master cylinder and the cylinder that applies the friction at each wheel is known as a caliper or wheel cylinder. This is referred to as the hydraulic system. No matter how long the lines are, as long as they are full of fluid there will be immediate movement at the output for every movement you make at the input.

The basic principles of hydraulics are indicated here. If water is forced from one into the other, movement takes place that is identical as long as no air is present. Notice that the blue cylinder’s piston is down and the pink cylinder’s piston is up.

When the pink cylinder’s piston is pressed down, the blue cylinder’s piston comes up the exact same amount. No matter how long the brake line or hose connecting the two cylinders is, the same movement takes place if the cylinders are the same diameter and no air is present. If the input cylinder is smaller than the output, the output cylinder moves less but has greater force. This is how the hydraulic circuit between the master cylinder and a brake caliper operates.

A caliper contains a cylinder with at least one piston that clamps two brake pads around the rotor. A wheel cylinder is a cylinder with a piston at each end that push the two brake shoes into the spinning drum. In both cases, the pistons press the friction material against the spinning component at each wheel to stop the vehicle.

Steel tubing and high-pressure hoses route the pressurized fluid to each wheel. The fluid must not contain any air so that all the input can travel to the outputs. Fluid cannot compress but air can and it affects brake force and pedal height. If there’s air in the system, the brake pedal feels spongy and may drop because you are compressing the air when you press down. Brake bleeding is the term used for the process of expelling air from the system.

Understanding the Pieces

The following is a description of each component in the brake system.

In a real brake system, high-pressure cylinders and lines are used, but the concept is no different than in the illustrations. The input piston in the master cylinder is applied when you press on the brake pedal and fluid flows to pistons of different sizes to produce the force needed at that wheel. Disc brakes require high force, so a larger piston is used. Drum brakes require less force but more movement to apply them, so a smaller piston is used.

The entire hydraulic system is filled with brake fluid and linked with steel tubing and high-pressure hoses and must be absent of air because air can compress, causing ineffective braking and a low, spongy brake pedal.

Brake Pedal

The brake pedal is on a pivot pin and lever action is used to increase the force applied by your foot. The linkage or rod from the pedal to the power booster or master cylinder can be adjusted. The brake light switch, or pedal travel sensor, is often located near the pedal lever to monitor its position. It turns on the brake lights and may also be an input to various other systems on the vehicle such as ABS, cruise control, or an electronic brake module.

The brake pedal is a lever that presses on a rod leading to the master cylinder. Mechanical advantage is obtained because the lever is offset. Most brake pedal linkage is adjustable. If the vehicle has power brakes, a power booster is located between the brake pedal and master cylinder to increase the force and reduce pedal effort.

Power Booster

The booster is mounted on the bulkhead (firewall) and uses vacuum or power steering pressure to increase the pedal force going into the master cylinder. The input to the booster is supplied by mechanical foot force from the brake pedal rod on the other side of the firewall. The output is transmitted through a pushrod into the master cylinder. Sometimes this pushrod is adjustable and the proper length is critical to prevent brake lockup or excessive pedal travel.

The power booster uses vacuum or power steering pressure to increase the force from the brake pedal to the master cylinder. Most modern vehicles use power brakes, which greatly reduces the effort needed to apply the brakes. The engine must be running for the power brakes to function. Should the engine die, a backup system ensures at least two assisted stops. The master cylinder receives the force from the power booster (arrow) and uses pistons to force brake fluid under pressure to the wheels.

Master Cylinder

The master cylinder bolts to the booster and contains two pistons that move when the brake pedal is applied. One piston supplies fluid to two wheels and the other piston serves the other two wheels. A reservoir of fluid above the pistons supplies reserve fluid for the system. When your foot is off the brake, the master cylinder allows fluid to flow from the wheels into the reservoir. This is necessary because the fluid expands when it heats up and must be allowed to escape from the wheels or the brakes may lock up. The passages inside the master cylinder responsible for this fluid escape are called vent ports.

Brake Lines

Metal brake lines lead from the master cylinder to flexible hoses at the wheels. Some vehicles have hydraulic valves between the master cylinder and the wheels. Most modern vehicles also incorporate an ABS modulator in the lines between the master cylinder and the wheels, which is controlled by a processor.

Brake lines are usually made of high-strength steel to withstand hydraulic pressures that can exceed 1,600 pounds. Special fittings are used at each connection to ensure safety and eliminate leakage. Lines are subjected to harsh operating conditions under the vehicle, so rust is a common problem that can lead to the need to replace the lines.

Older vehicles used mechanical valves to vary the hydraulic pressure to the wheels during braking. A proportioning valve in the line to the rear brakes reduces rear wheel lockup during panic stops and a metering valve in the line to the front brakes delays the front disc brakes long enough for the rear drum brakes to apply. This reduces nosedive during braking. A pressure differential switch turns on the red brake warning lamp if a leak occurs. Most of these functions are performed electronically on current models.

Brake Hoses

At the end of the metal lines, rubber brake hoses are used to allow for wheel movement during steering and suspension operation. These hoses connect the metal lines to the calipers or wheel cylinders and route the high-pressure fluid to the calipers and wheel cylinders during brake operation.

Flexible brake hoses are used between the steel brake lines and the wheels to accommodate steering and suspension movement. These hoses contain multiple layers of tubing and high-strength fabric to withstand the high hydraulic pressures in the system. Careful inspection is required to determine if cracks have developed, which are an indication that replacement is needed.

Brake Calipers

A brake caliper contains at least one piston that converts hydraulic pressure from the lines and hoses to the mechanical movement needed to force the brake pads into the spinning rotor, which slows or stops the vehicle. When the brakes are released, the piston seal slightly retracts the piston to reduce drag and improve fuel economy.

A brake caliper contains one or more pistons that clamp the brake pads against the spinning rotor when the brakes are applied. It contains anti-rattle hardware to reduce brake noise. Most calipers slide or float on pins or ledges that must be clean and lubricated to ensure proper operation.

Wheel Cylinders

Wheel cylinders contain pistons that convert the hydraulic pressure from the lines and hoses to mechanical force to apply the brake shoes to the inside of the drum to slow the vehicle. When the brakes are released, springs inside the drum pull the shoes away from the drum.

A wheel cylinder is used to apply the brake shoes inside of a drum brake system. It contains two pistons that extend outward due to the hydraulic pressure generated when the brake pedal is applied. This movement is transferred to the brake shoes, which are forced into the spinning drum.

Brake Pads

Brake pads have friction material attached to a plate of metal. A brake pad is located on each side of the rotor. One is known as the inboard pad (nearest the piston) and the other is known as the outboard pad. They are squeezed with high force against the rotor by the caliper piston(s) when the brakes are applied.

Brake pads are relatively small sections of friction material attached to a metal backing plate. They are placed on each side of the rotor and clamped against it by the caliper when the brakes are applied. They provide very effective braking and handle heat well.

Disc Brake Hardware

Disc brakes use metal slides, shims, and anti-rattle clips to minimize brake noise and ensure proper operation during brake apply and release. Some are insulated on rubber bushings.

Because brake squeal is a byproduct of the vibration caused by brake systems, engineers generally use anti-rattle clips and springs to minimize the noise generated by disc brake systems. A shim between the brake pad and caliper or piston may also be used to reduce the potential for brake noise.

Brake Shoes

Brake shoes are C-shaped and are lined with friction material. The two inside of each drum are forced against the inside of the brake drum when the brakes are applied. They are also sometimes referred to as brake linings.

Brake shoes perform the same task as brake pads. Friction material is attached to the shoe with rivets or a bonding process. When the shoes contact the drum during braking, they increase their force by wedging into the drum.

Drum Brake Hardware

Several springs and linkages are used to keep the brake shoes in proper position and also serve to retract the brake shoes when the brake pedal is released. In addition, automatic adjusters are used to maintain proper brake pedal height as the shoe material wears. Some of the hardware is also used to apply the parking brake when the vehicle is parked.

Drum brakes use hold-down and return springs as well as automatic adjuster and parking brake components. These components are part of a hardware and adjuster kit when a brake overhaul is needed. Due to heat, these components should be replaced during servicing.

Rotors

Brake rotors are connected to the wheels and rotate as the wheels turn. When the brake pads clamp around the rotor, the rotor is slowed or stopped. Heat is generated and must be released through conduction and radiation.

Brake rotors are made of iron and generally have cooling fins when used on the front of a vehicle. Some rear rotors are solid without cooling fins. The brake pads are squeezed around the rotor surface by the caliper when the brakes are applied, which slows or stops the vehicle.

Drums

The brake drums also have the vehicle’s wheels bolted to them and they rotate as the wheels turn. When the brake shoes are pressed into the inside of the drum during brake operation, the drum is slowed or stopped.

Brake parts on older cars were made of heavy cast iron and steel. Modern cars use more aluminum and plastic. Due to reduced mass, brake noise is more common today than ever. It is imperative to do the job right using quality parts or brake noise and other problems may occur.

The brake drum covers the shoes that expand into it when the brakes are applied. Because the drum fits over the wheel studs, this action slows the vehicle.

Beyond the Basics

In addition to these basic components found in nearly every brake system, the vehicle could also be equipped with various valves and ABS components. As systems have progressed, the brake system is the basis for several other systems including the traction control system (TCS), vehicle stability control (VSC), ABS, ADAS, and stability control. These systems will continue to be key components of future vehicles, even autonomous vehicles. Chapter 9: ABS, ADAS, and Related Systems contains more information on these systems.