ASE T4 Brakes (Medium/Heavy Truck) flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Governor cut-out pressure

The pressure at which the compressor governor signals the unloader to stop compression, typically 120 to 135 psi in a heavy truck air system.

Low-pressure warning activation

A dash lamp and buzzer that must activate when system air pressure drops to 60 psi or below, warning the driver of insufficient braking pressure.

Air dryer purge cycle

Occurs at governor cut-out, when the purge valve opens and exhausts collected moisture and oil from the desiccant cartridge out to atmosphere.

Safety valve setting

A spring-loaded pop-off valve set near 150 psi that vents the wet tank if the governor fails, preventing reservoir rupture from overpressure.

Compressor unloader

A mechanism actuated by governor air pressure that holds the compressor inlet valves open so no air is pumped once cut-out pressure is reached.

Supply check valve

A one-way valve allowing flow from compressor and dryer into the wet tank while blocking backflow, so a failed compressor cannot drain the reservoirs.

Automatic slack adjuster

A self-adjusting lever that maintains correct pushrod stroke by ratcheting during the return stroke and must be checked for free play at full application.

Pushrod stroke limit

The maximum legal travel of a brake chamber pushrod, which varies by chamber size and type, such as 2 inches for a clamp-type Type 30 chamber.

S-cam brake operation

Air pressure pushes the pushrod out, rotating the slack adjuster and S-cam, whose lobes force the brake shoes outward into contact with the drum.

Brake drum maximum diameter

The maximum allowable inside diameter stamped on the drum; turning or wear beyond it reduces lining contact and heat capacity and requires replacement.

Ruptured chamber diaphragm

A failure causing a continuous air leak, weak or no braking at that wheel, and slow system pressure buildup affecting the whole vehicle.

Broken shoe return spring

A failed spring that no longer pulls the shoes off the drum, causing dragging, overheating, and rapid lining wear at that wheel.

Wheel speed sensor

A variable-reluctance sensor at each wheel that generates an AC voltage whose frequency and amplitude rise with wheel speed for the ABS ECU.

ABS modulator valve

An electrically controlled air valve with apply, hold, and release states that adjusts pressure to an individual wheel during a slip event.

Automatic traction control function

ATC reduces engine torque and applies drive-wheel brakes when the ABS ECU detects wheel spin during acceleration, improving traction on slippery surfaces.

ABS warning lamp

Illuminates briefly at key-on as a bulb check and stays on when the ECU stores an active fault; blink codes are retrieved per manufacturer procedure.

ABS sensor air gap

The clearance between wheel speed sensor tip and tone ring, typically 0.010 to 0.040 inch; wrong gap or debris causes intermittent or absent signal.

DOT 5 brake fluid

A silicone-based fluid that is not compatible with glycol fluids such as DOT 3, DOT 4, and DOT 5.1; mixing them causes seal swelling and brake failure.

Spongy brake pedal

A pedal that feels soft and travels too far, usually caused by air in the hydraulic system and corrected by bleeding at each wheel.

Metering valve

In disc/drum systems, it holds off front disc pressure momentarily so rear drum shoes can overcome return spring force, balancing brake timing.

Proportioning valve

Limits hydraulic pressure to the rear brakes during hard stops, reducing rear-wheel lockup and helping maintain directional stability.

Sinking brake pedal

A pedal that slowly falls under steady pressure indicates an internal master cylinder cup seal leak bypassing fluid, requiring master cylinder repair or replacement.

Spring brake chamber

A dual-chamber unit combining a service diaphragm with a mechanically stored power spring that applies the brakes when air pressure is released.

Parking control valve

The yellow dash valve that exhausts air from spring brake chambers to apply the parking brake and admits air to release it.

Caging bolt

A manual release bolt that compresses the power spring so a spring-braked vehicle can be moved; the spring brakes are inoperative while it is installed.

Inversion valve

A relay valve that supplies a modulated spring brake application when service brake pressure is lost, allowing a controlled emergency stop.

Anti-compounding valve

Prevents service brake air pressure and spring brake force from applying at the same time, avoiding damage to foundation brake components.

Wheel bearing end play

The slight axial clearance after bearing adjustment, commonly 0.001 to 0.005 inch; too little causes overheating and too much causes seal leakage.

Wheel seal journal

The smooth surface on the spindle that the seal lip rides on; a groove or scratch there causes repeated lubricant leaks even with a new seal.

Bearing cone and cup

The cone is the inner race assembly with rollers and the cup is the outer race pressed into the hub; they are replaced as matched sets.

Wet hub lubrication

A hub design that carries gear oil, sometimes shared with the differential; the specified lubricant must be used and never mixed with grease.

Bearing spalling

Flaking or pitting of bearing race surfaces caused by fatigue, contamination, or inadequate lubrication, producing noise and requiring bearing replacement.

Contaminated brake lining

Lining coated with hub oil or grease cannot be cleaned back to service condition and must be replaced along with repairing the leak source.