ASE A5 key terms

The hydraulic liquid that transmits force from the master cylinder to the wheel brakes. Glycol-based DOT 3 and DOT 4 fluids absorb moisture over time and must be replaced periodically.

A piston pump mounted on the firewall that converts brake pedal force into hydraulic pressure. Its reservoir supplies fluid and its ports allow fluid to return when the pedal is released.

The procedure for removing air from a master cylinder before installation, usually by stroking the piston while outlet ports are submerged in fluid or connected to recirculating hoses.

A hydraulic valve that reduces pressure to the rear brakes during hard stops so the rear wheels do not lock before the front wheels.

A sensor inside a combination valve that detects unequal pressure between the front and rear hydraulic circuits and illuminates the red brake warning lamp.

A check valve that keeps a small amount of pressure in drum brake circuits to prevent wheel cylinder cup seepage. It is generally not used with disc brakes because it can cause pad drag.

The process of purging trapped air from the hydraulic system by forcing fresh fluid through the lines until no bubbles appear at the bleeder screws.

The small amount of pedal movement before the pushrod contacts the master cylinder piston. Correct free play keeps the compensating port open so fluid can return to the reservoir.

A flexible reinforced rubber line that carries hydraulic pressure from the body lines to a wheel that moves with suspension and steering. It must not balloon, crack, or leak.

A hydraulic cylinder bolted to the backing plate that pushes the brake shoes outward against the drum when pressure is applied.

The rotating cast-iron or steel drum attached to the wheel hub. Brake shoes press against its inner machined surface to create friction and slow the wheel.

A curved metal platform with bonded or riveted friction lining that presses against the drum. Shoes are positioned as primary or secondary and must be installed correctly.

A drum brake action in which the rotating drum drags the leading shoe into tighter contact with the drum, multiplying apply force and reducing required pedal effort.

A ratcheting threaded link between drum brake shoes that automatically maintains proper lining-to-drum clearance as the friction lining wears thinner.

A temporary loss of stopping power caused by heat, such as lining outgassing or brake fluid boiling, which reduces friction or hydraulic effectiveness during repeated hard stops.

The hydraulic clamp that straddles the rotor and presses the brake pads against it. It may be fixed with opposed pistons or floating on lubricated slide pins.

The rotating cast-iron disc that the brake pads squeeze to slow the wheel. Its thickness, surface finish, and runout directly affect stopping smoothness and pedal feel.

Variation in rotor thickness around its circumference that causes alternating brake torque and is a common cause of pedal pulsation during braking.

Side-to-side wobble of a brake rotor as it rotates, measured with a dial indicator. Excessive runout can push the pads back and cause pulsation or a low pedal.

Lubricated guide pins that allow a floating caliper to move side to side so both brake pads contact the rotor evenly. Sticking pins can cause drag or uneven pad wear.

The friction material bonded to a steel backing plate that is pressed against the rotor to create stopping friction. Pads often include shims, insulators, and wear indicators.

A large diaphragm chamber that uses engine vacuum and atmospheric pressure to multiply brake pedal force, reducing the effort required to stop the vehicle.

A one-way valve in the booster vacuum supply line that traps vacuum inside the booster so power assist remains available for one or more stops after the engine shuts off.

A power assist unit that uses pressurized power steering fluid rather than engine vacuum to multiply brake pedal force. It is common on diesel and heavy-duty vehicles.

A pressure storage device in a hydro-boost or ABS system that holds pressurized fluid to provide reserve assist or rapid brake pressure modulation when needed.

The mechanical advantage between the pedal pad and the pushrod pivot. A higher ratio increases force at the master cylinder but also increases pedal travel.

A bearing that supports the wheel hub and allows it to rotate with minimal friction. It may be a serviceable tapered roller set or a sealed hub unit.

The small axial movement of a wheel hub allowed by bearing clearance. Too much play can cause wobble, noise, and uneven brake pad or tire wear.

A mechanical parking brake assembly with small shoes inside a drum formed in the center of a rear disc rotor. It is applied by a cable or electric actuator.

A switch at the brake pedal that turns on the stop lamps and signals the ABS, engine, or transmission control module when the driver applies the brakes.

The computer that monitors wheel speed sensors and commands the hydraulic modulator to hold, release, and reapply brake pressure and prevent wheel lockup.

A magnetic or hall-effect sensor at each wheel that produces a signal proportional to wheel rotation speed for ABS, traction control, and stability control systems.

A toothed or magnetically encoded ring on a hub or axle that rotates past the wheel speed sensor to generate the wheel speed signal.

The ABS valve body containing solenoid valves and a pump that can hold, release, or reapply brake pressure at individual wheels under module control.

A solenoid in the ABS modulator that closes to seal off the driver's brake pressure from a wheel so the system can release pressure without losing pedal.

A solenoid in the ABS modulator that opens to release brake pressure from a wheel to the accumulator or reservoir, allowing a locked wheel to spin back up.

A system that uses ABS wheel speed sensors and brake or throttle intervention to reduce wheelspin during acceleration on low-traction surfaces.

A system that compares steering angle, yaw rate, and wheel speeds and applies individual brakes or reduces engine torque to help correct a skid.

A software function that varies rear brake pressure using ABS components to balance braking and reduce rear lockup, often replacing a conventional proportioning valve.