ASE H4 Brakes (Transit Bus) key terms

A sealed canister containing a diaphragm that converts compressed air pressure into mechanical force to apply a service brake. It is mounted on the axle and pushes a pushrod when air enters.

A flexible rubber disc inside a brake chamber that separates the pressurized air side from the atmospheric side. Air pressure flexes it, which moves the pushrod to apply the brake.

The rod that extends from a brake chamber when air pressure flexes the diaphragm. Its stroke length and travel indicate brake adjustment and chamber condition.

The linear distance the pushrod travels from its released position to its fully applied position. Excessive stroke signals worn linings or out-of-adjustment brakes and must be corrected.

The linkage between the brake chamber pushrod and the brake camshaft that transfers force and allows adjustment of lining-to-drum clearance. It may be manual or automatic.

A self-adjusting slack adjuster that takes up clearance as linings wear during normal brake applications. It reduces the need for manual adjustment but still requires inspection.

A drum brake design in which an air-applied camshaft rotates an S-shaped cam that spreads the brake shoes against the drum. It is common on heavy transit bus axles.

A curved metal carrier holding friction lining that presses against the inside of a brake drum to create stopping friction. Shoes pivot on anchor pins and are spread by the cam.

The friction material bonded or riveted to a brake shoe or pad. It wears with use and its thickness is a key inspection point for brake service.

The rotating cast-metal cylinder attached to the wheel hub that the brake shoes press against. Its inside surface provides the friction face and can wear, score, or crack.

An engine-driven pump that draws in air and compresses it to supply the vehicle's air brake system. It is typically cooled and lubricated by the engine and controlled by a governor.

A device that senses reservoir pressure and controls the compressor's loading and unloading. It cuts the compressor in at a low setpoint and out at a high setpoint to maintain working pressure.

The upper reservoir pressure at which the governor unloads the compressor so it stops building pressure. Typical values are around 120 to 135 psi depending on the system.

The lower reservoir pressure at which the governor lets the compressor load and resume building pressure. It is normally set about 15 to 25 psi below cut-out pressure.

A device downstream of the compressor that removes moisture and contaminants from compressed air before it reaches the reservoirs. It usually contains a desiccant cartridge and a purge cycle.

A storage tank that holds compressed air for brake application and auxiliary use. Tanks are divided into wet, primary, and secondary sections to protect brake circuits from moisture and loss.

The first reservoir that receives air directly from the compressor and dryer, where most moisture condenses. It supplies air to the other circuits and is drained regularly.

A one-way valve that permits air to flow in one direction only. In air systems it prevents backflow and protects charged reservoirs from losing pressure when supply drops.

A valve with two inlets and one outlet that delivers whichever input pressure is higher to the outlet. It is used to shuttle between two air sources safely.

A valve mounted near the brakes that uses a small control signal to admit reservoir air directly to the brake chambers. It shortens application time and improves release on long vehicles.

A valve placed near brake chambers that exhausts applied air rapidly when the control pressure is released. It speeds brake release and reduces lag on long wheelbases.

The treadle-operated brake valve the driver presses to apply service brakes. It meters air pressure to the brake circuits in proportion to pedal travel and force.

The pedal plate and linkage of the foot brake valve that the driver depresses. Its travel and feel are checked during brake application and leak testing.

A parking brake chamber that uses a heavy spring to apply the brake when air pressure is released. Air pressure compresses the spring to release the brake for normal driving.

A dash-mounted push-pull valve that controls air to spring brake chambers. Pulling it exhausts air to apply the parking brake; pushing it admits air to release them.

A valve that prevents service brake air and spring brake air from applying at the same time in a spring brake chamber. It protects the chamber from combined excessive force.

A sensor at each monitored wheel that reports rotational speed to the ABS electronic control unit. It detects wheel lockup so the system can modulate brake pressure.

A toothed ring that rotates with the wheel and generates a signal in the wheel speed sensor. Damaged or missing teeth cause erratic speed signals and ABS faults.

The electronic control unit that processes wheel speed signals and commands the modulator valves. It also stores diagnostic trouble codes and controls the ABS warning lamp.

An electrically controlled air valve that the ABS ECU uses to hold, release, or reapply brake pressure at a wheel. It prevents wheel lockup during hard braking.

A flashing-lamp diagnostic pattern the ABS ECU uses to report stored trouble codes. Technicians count the flashes to identify faults when a scan tool is unavailable.

A stored alphanumeric code that identifies a specific fault detected by an electronic control system. It guides troubleshooting but must be verified with physical tests.

A sensor that triggers a warning light and buzzer when reservoir pressure falls below a safe level. It alerts the driver before braking performance is compromised.

A procedure that charges the air system, stops the compressor, and observes pressure loss over time. It reveals leaks in the compressor, lines, valves, or brake chambers.

Setting the clearance between friction material and drum or rotor so pushrod stroke stays within specification. Proper adjustment keeps stopping performance and chamber travel correct.

The process of removing trapped air from a hydraulic brake system by forcing fluid through the lines. Air in the system causes a spongy pedal and reduced braking.

A hydraulic pump driven by the brake pedal that pressurizes brake fluid and sends it to the wheel circuits. It also contains reservoirs and may include a proportioning function.

A hydraulic actuator at a drum brake that converts fluid pressure into movement of the brake shoes. Leaking seals here cause fluid loss and contaminated linings.

A hygroscopic hydraulic fluid that transmits pedal force through the brake system. It has a specified boiling point and must meet the manufacturer's rating.