ASE S4 Brakes (School Bus) key terms

A glycol-ether based hydraulic brake fluid with a minimum dry boiling point of about 401°F (205°C). It absorbs moisture over time, which lowers its boiling point and can cause brake fade if not changed.

The temperature at which brake fluid turns to vapor in the hydraulic system. Vapor is compressible, so a boiling fluid causes a spongy or low pedal and reduced stopping power during hard use.

Loss of braking effectiveness caused by heat. It can come from overheated friction material (lining fade) or from vapor bubbles in hot brake fluid (fluid fade). Both produce a weak or soft pedal response.

The pump that converts driver pedal force into hydraulic pressure. It holds a reservoir of fluid and uses a piston and cup seals to push fluid into the brake lines toward the wheel units.

The process of removing trapped air from a master cylinder on the bench before installation. It is done by stroking the piston while ports are plugged or routed back into the reservoir until no bubbles appear.

The procedure of purging air from the hydraulic system by opening bleeder screws at each wheel while an assistant holds pressure on the pedal or a pressure bleeder is applied, until clear bubble-free fluid flows.

A small hollow screw with a tapered seat, located at the highest point of a wheel cylinder or caliper. Loosening it lets trapped air and old fluid escape during bleeding.

A hydraulic device that limits pressure to the rear brakes during hard stops, helping prevent rear wheel lockup and maintaining balanced braking between front and rear axles.

An assembly that houses the proportioning valve along with a metering (hold-off) valve and a pressure differential switch, commonly used on front-disc/rear-drum hydraulic systems.

A safety switch that senses unequal pressure between the front and rear hydraulic circuits, such as after a leak. It triggers a red brake warning lamp on the dash.

A dash indicator that lights when the parking brake is applied, when fluid level is low, or when the pressure differential switch detects a hydraulic imbalance. It warns of a system fault.

The translucent plastic tank mounted on or near the master cylinder. It stores fluid for the hydraulic system and is marked with MIN and MAX levels that should be checked with the cap clean.

The hydraulic actuator inside a drum brake that pushes the brake shoes outward against the drum when pressure is applied. It contains pistons, cup seals, and a bleeder screw.

The hydraulic clamp that straddles a disc and presses the brake pads against it. It may be fixed or floating, and it contains one or more pistons sealed by a square-cut O-ring.

A square-cut rubber ring that seals the caliper piston and also retracts it slightly when pressure is released, providing running clearance between pad and rotor without a return spring.

The friction material bonded to a steel backing plate that presses against the rotor to slow the vehicle. Pads are rated for temperature and must be replaced in axle sets.

The rotating disc that brake pads clamp against. It must meet a minimum thickness specification and be free of excessive runout, scoring, or cracks to provide smooth, safe braking.

Side-to-side wobble of a rotor or drum as it turns, measured with a dial indicator. Excessive runout causes pedal pulsation and uneven pad wear and can push the caliper pistons back.

The rotating cast-iron housing that brake shoes press against from the inside. It has a maximum inside diameter limit, and oversize or out-of-round drums cause poor or grabbing braking.

A curved metal platform with bonded or riveted friction lining that is pushed outward against a drum by the wheel cylinder. Shoes are installed in axle sets with matched linings.

A mechanism in a drum brake that turns the adjusting star wheel as the vehicle backs up or as the shoe travel increases, keeping shoe-to-drum clearance within specification.

A valve in some master cylinders or line fittings that holds a small pressure in the lines, keeping drum wheel cylinder cups seated. It must not be used with disc brakes.

The engine-driven pump that supplies compressed air to the air brake system. It is cooled and lubricated and is controlled by a governor that unloads it at the cut-out pressure.

A device mounted on the compressor that senses reservoir pressure and controls when the compressor loads and unloads. Cut-out is typically around 120-125 psi and cut-in about 20-25 psi lower.

The low and high reservoir pressures at which the governor loads and unloads the compressor. The spread between them is the normal operating range of the air brake system.

A device between compressor and reservoir that removes moisture and oil from compressed air using a desiccant cartridge. It has a purge cycle and a heater to prevent freeze-up.

A storage tank for compressed air, also called a wet or dry tank. It supplies air for braking and accessories and is fitted with drain valves to remove condensed water.

An optional device that injects alcohol vapor into the air system to lower the freezing point of condensed water in lines and valves during cold weather operation.

The treadle-operated valve the driver pushes to apply the air brakes. It meters air pressure from the reservoirs to the brake chambers in proportion to pedal travel.

A large-capacity valve mounted near the rear axle that uses a small control signal from the foot valve to deliver a large volume of air quickly to the rear brake chambers.

The air cylinder that converts air pressure into mechanical force to apply the brakes. A spring brake chamber also contains a strong spring that applies the parking brake when air is released.

A parking and emergency brake mechanism inside a brake chamber that uses a compressed coil spring to apply the brakes when air pressure is removed. Air pressure releases it to allow movement.

A long threaded bolt used to mechanically compress and lock the spring in a spring brake chamber so the vehicle can be moved when air pressure is unavailable. It must be removed before service.

The lever that links the brake chamber pushrod to the brake shoe camshaft. Its length is adjusted so pushrod travel stays within specification, keeping brake timing correct.

The distance the brake chamber pushrod extends during a full brake application. Excessive travel indicates worn shoes or out-of-adjustment slack adjusters and must be corrected.

The dash-mounted yellow push-pull valve that controls the spring brakes. Pulling it exhausts air to apply the parking brake; pushing it admits air to release the brakes.

An electrical switch actuated by the parking brake control or mechanism that completes the circuit to the red brake warning lamp when the parking brake is applied.

A hydraulic power assist system that uses pressurized power steering fluid, stored in an accumulator, to boost brake pedal force. It provides assist even if the engine stalls.

A pressure storage device in a hydraulic assist system that holds pressurized fluid or nitrogen so several power-assisted stops remain available if the pump or engine stops.

A power assist unit that uses engine vacuum acting on a diaphragm to reduce pedal effort. A failed check valve or leaking diaphragm causes a hard pedal and rough idle.

A one-way valve that allows flow in only one direction. In brake systems it may retain vacuum in a booster or control air flow between components, and a stuck valve causes assist loss.

A sensor at each wheel that reports wheel rotational speed to the ABS module. A damaged or dirty sensor produces an ABS warning lamp and disables antilock function.

A toothed or magnetic ring that rotates with the wheel and generates a signal in the wheel speed sensor. Missing teeth or debris cause erratic speed readings and false ABS activation.

The electronic control unit that monitors wheel speed signals and commands the hydraulic modulator to release and reapply brake pressure, preventing wheel lockup during hard stops.

The hydraulic valve body that the ABS module uses to hold, release, and reapply pressure at individual wheels. It contains solenoid valves and a pump and must be bled per service procedure.

A dash lamp that illuminates when the ABS module detects a fault and disables antilock operation. Normal service brakes usually remain functional, but the fault must be diagnosed.