ASE H4 Brakes (Transit Bus) flashcards
ASE H4 Brakes (Transit Bus) · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Dual-circuit air brake system
- An air brake arrangement with two independent reservoirs and circuits so that a failure in one circuit still leaves at least one service brake circuit operable.
- Relay valve function
- A valve that uses a small control signal from the foot valve to supply reservoir air directly to remote brake chambers, speeding application and release.
- Quick-release valve
- A valve mounted near brake chambers that exhausts chamber air locally at release, reducing lag time on long wheelbase vehicles.
- Spring brake chamber
- A brake chamber containing a large stored spring that applies the parking brake; air pressure compresses the spring to release the brake.
- Safety (inversion) valve
- A valve on a spring brake chamber that prevents spring release if the service side is pressurized without the spring cavity pressurized.
- Pushrod stroke measurement
- The distance from the brake chamber face to the center of the clevis pin at full application; excessive stroke signals adjustment or component wear.
- Governor cut-out pressure
- The reservoir pressure at which the governor unloads the compressor, typically about 120 to 135 psi, stopping further pressure buildup.
- Air dryer function
- A device that removes moisture and oil aerosol from compressor discharge; a failed desiccant or purge valve causes freeze-ups and valve corrosion.
- Safety relief valve
- A reservoir-mounted valve that opens near 150 psi to protect the system from overpressure; it must not be used as a pressure regulator.
- Alcohol evaporator
- A device that injects alcohol vapor into the compressor discharge to lower the dew point and prevent air line freeze-up in cold weather.
- Compressor build-up time
- The time required to raise reservoir pressure from cut-in to cut-out; slow build-up indicates a worn compressor, leaks, or a stuck unloader.
- Low-pressure warning switch
- A switch that activates the buzzer or warning light when system pressure falls below about 60 psi, alerting the driver to low air pressure.
- Pascal's law
- The principle that pressure applied to a confined fluid is transmitted equally in all directions, so equal pressure acts on all wheel cylinder pistons.
- Hydraulic force formula
- Force equals pressure multiplied by piston area; a larger wheel cylinder piston produces greater braking force for the same line pressure.
- DOT 3 and DOT 4 fluid
- Glycol-based hydraulic brake fluids that are hygroscopic; absorbed moisture lowers boiling point and promotes corrosion, requiring periodic replacement.
- DOT 5 silicone fluid
- A silicone-based brake fluid that is not compatible with glycol fluids; mixing causes seal swelling and loss of braking.
- Residual pressure check valve
- A valve in the master cylinder that maintains slight pressure to drum wheel cylinders to keep cup seals seated.
- Metering (hold-off) valve
- A valve that delays front disc brake application until rear drum brakes begin to apply, improving balance during light stops.
- Proportioning valve
- A valve that reduces rear hydraulic pressure to prevent rear wheel lockup during hard braking.
- Caging a spring brake
- Manually compressing the spring brake with the caging bolt so the vehicle can be moved or the chamber removed without air pressure.
- Automatic slack adjuster
- A slack adjuster that adjusts during the return stroke when pushrod stroke exceeds its internal threshold, maintaining correct brake adjustment.
- Manual slack adjuster setting
- Turning the adjusting hex until the lining contacts the drum, then backing off to the specified clearance.
- Parking brake will not release
- With air applied, a parking brake that stays applied indicates a faulty release valve, stuck relay valve, or seized spring brake chamber.
- Wheel speed sensor signal
- An AC signal whose frequency is proportional to wheel speed; a damaged tone ring or incorrect air gap sets an ABS fault code.
- ABS modulator valve
- A solenoid-controlled valve that holds, releases, or reapplies pressure to a brake chamber independently of the driver's foot valve.
- ABS relay valve
- A valve that supplies reservoir air to the ABS modulator valves and is controlled by the ABS electronic control unit.
- Blink-code diagnostics
- A diagnostic method on older ABS systems that counts flashes of the ABS warning lamp to identify stored fault codes.
- Brake drag diagnosis
- Comparing hub temperatures after a road test; a hotter wheel indicates a dragging brake, restricted line, or faulty hardware.
- Air brake leak-down test
- With the system charged and engine off, pressure should not drop more than the manufacturer's limit over the specified time.
- Brake pull diagnosis
- A pull to one side indicates a dragging brake, contaminated lining, or a restricted hydraulic line on the opposite side.
- Rotor thickness and runout
- Rotors are measured for thickness and runout; machining is permitted only if finished thickness stays above the minimum specification.
- Bleeding sequence
- Hydraulic systems are bled in the manufacturer's specified order, usually starting at the wheel farthest from the master cylinder.