ASE T4 Brakes (Medium/Heavy Truck) key terms

A pressure-sensing valve mounted on the air compressor that stops the compressor from pumping at cut-out pressure and lets it resume at cut-in pressure, keeping reservoir pressure within a set range.

A belt- or gear-driven pump that draws in filtered air and compresses it into the supply reservoir, usually routing it through the air dryer first to remove moisture.

A canister between the compressor and the first reservoir that traps water vapor and oil in a desiccant bed, then blows the collected contaminants out to atmosphere during its purge cycle.

The first tank to receive air from the compressor and dryer, often called the wet tank; it collects condensation and feeds the primary and secondary circuits through one-way check valves.

A valve that permits air to flow in only one direction, used to keep pressure trapped in a reservoir so a leak or failure elsewhere does not drain the whole system.

The treadle valve the driver operates with the brake pedal; it meters air from the reservoirs into the service circuits in proportion to pedal travel, then exhausts it on release.

A valve mounted close to the brakes that uses a small control signal from the foot valve to quickly supply and exhaust a large volume of air to the brake chambers.

A valve that closes automatically when a trailer separates or a supply line breaks, holding air in the tractor reservoirs and causing the trailer spring brakes to apply.

A sensor that closes a circuit when system air pressure drops below a set value, lighting a dash lamp and sounding a buzzer so the driver stops before braking is lost.

The lever on the S-cam shaft that turns pushrod force into cam rotation and takes up clearance as linings wear, keeping pushrod stroke within the legal range.

A drum brake in which an S-shaped cam rotates and spreads two shoes outward against the drum; the cam is turned by a slack adjuster and pushrod.

A cylinder containing a diaphragm and pushrod that turns air pressure into mechanical force, applying the service brake when the foot valve sends air to it.

A combination chamber with a service diaphragm plus a large stored-energy spring that applies the parking brake when air is released; the spring can be caged for manual release.

The distance a brake chamber pushrod travels from the released position to full application; stroke beyond the manufacturer's limit means the brake is out of adjustment.

Friction material bonded or riveted to a curved steel shoe that is pressed against the drum or rotor, converting vehicle motion into heat to slow the wheel.

The rotating cast-iron or steel part bolted to the wheel hub; the brake shoes rub against its machined inner surface to create stopping force.

An assembly that clamps pads against a rotor, driven by an air chamber through an internal adjuster; it resists fade better and holds adjustment longer than drum brakes.

The electronic control unit that reads wheel speed signals, detects an impending wheel lockup, and commands the modulator valves to hold, release, or reapply pressure.

A magnetic or active sensor mounted at each wheel that produces a signal proportional to wheel rotation, allowing the ECU to calculate wheel speed and slip.

A toothed ring pressed onto a hub or rotor that turns past the wheel speed sensor, generating a pulsed signal the ECU uses to measure wheel speed.

An electrically controlled valve that can hold, release, or reapply air pressure to one brake chamber on command from the ECU during an antilock event.

A system that uses the ABS wheel sensors to detect drive-wheel spin and then reduces engine torque and/or applies the brakes to restore grip.

A flash pattern produced by an ABS warning lamp that identifies a stored trouble code, letting a technician diagnose faults when no scan tool is available.

The hydraulic pump driven by the brake pedal that pushes fluid into the lines, building pressure at the wheel cylinders or calipers when the pedal is depressed.

Hydraulic fluid rated by boiling point and base chemistry; DOT 3, 4, and 5.1 are glycol-based and DOT 5 is silicone, and silicone must never be mixed with glycol fluid.

A single housing that groups metering, proportioning, and pressure-differential warning functions, commonly used on vehicles with front disc and rear drum brakes.

A hold-off valve that delays front disc brake application until the rear drum brakes begin to work, preventing the front brakes from doing all the work at low pressure.

A valve that limits how much hydraulic pressure reaches the rear brakes during hard stops, reducing the chance of rear wheel lockup and loss of stability.

Removing trapped air from a hydraulic system by opening bleeder screws while forcing fluid through the lines, normally starting at the wheel farthest from the master cylinder.

A power-assist unit that uses pressurized power steering fluid rather than engine vacuum to reduce brake pedal effort; if it fails, pedal effort rises sharply but brakes still work.

The dash valve, marked with a yellow diamond knob, that exhausts air from the spring brake chambers to apply the parking brake and admits air to release it.

A valve that keeps service brake air pressure from adding to spring brake force when both are applied together, protecting the foundation brake from excessive load.

A long release bolt stored on each spring brake chamber that, when threaded in, mechanically compresses the parking spring so the vehicle can be moved with no air pressure.

A mechanical brake mounted on the transmission output or driveshaft that holds the vehicle when parked, using a small drum and shoes or a disc and caliper.

An antifriction bearing that supports the hub on the spindle; its tapered rollers carry both radial and axial loads and require correct end play or preload.

The cone is the inner race with rollers and cage; the cup is the outer race pressed into the hub. They are matched parts and should be replaced as a set.

A lip seal pressed into the hub that keeps lubricant in and dirt and water out at the spindle; a leaking seal can send oil onto brake linings and ruin them.

The small axial clearance set when adjusting a wheel bearing; too little clearance overheats and destroys the bearing, while too much allows wheel wobble and seal leakage.

A hub filled with gear oil that lubricates the wheel bearings, with level checked through a sight plug or fill hole; common on heavy-duty drive and trailer axles.

The stationary shaft end on the axle that the wheel bearings and hub ride on; it also provides the seal surface and the threads for the bearing adjusting nut.