ASE H4 Brakes (Transit Bus) cheat sheet
ASE H4 Brakes (Transit Bus) · Keep the key ideas within reach.
Air Brake System Components and Operation
A dual-circuit air brake system uses two independent reservoirs so a single circuit failure still leaves at least one service brake circuit operable.
The foot valve (treadle valve) meters air pressure to the service brake chambers in proportion to pedal travel; it is not an on/off switch.
A relay valve speeds application and release at remote chambers by using a small control signal to supply reservoir air directly to the brake chambers.
The quick-release valve exhausts chamber air locally at release, reducing lag on long wheelbase vehicles.
A spring brake chamber uses a large stored spring to apply parking/emergency braking; air pressure compresses the spring to release the brake.
The safety (inversion) valve on a spring brake chamber prevents release if the service side is pressurized without the spring cavity pressurized.
A front brake limiting (ratio) valve reduces front axle pressure at low delivery pressures to reduce jackknife tendency; it must be functioning for correct balance.
Pushrod stroke is measured from the brake chamber face to the center of the clevis pin at full application; excessive stroke signals adjustment or component wear.
Slack adjuster free play is checked by hand: the slack arm should move freely before the pushrod begins to move the shoe.
Air Supply and Compressor Systems
The compressor is typically engine-driven; a faulty unloader or governor keeps the compressor pumping continuously and causes pressure to exceed cut-out.
The governor controls compressor loading by sensing reservoir pressure: cut-out ~120-135 psi, cut-in typically 20-25 psi below cut-out.
The air dryer removes moisture and oil aerosol; a failed desiccant or purge valve sends moisture downstream and causes freeze-ups and valve corrosion.
The safety valve (relief valve) on the wet tank protects the system, typically opening near 150 psi; it must not be used as a pressure regulator.
The alcohol evaporator (where fitted) injects alcohol vapor into the compressor discharge to lower the dew point and prevent line freeze-up.
A low-pressure warning switch activates the buzzer/light below ~60 psi; a pressure gauge that reads system pressure confirms supply condition.
The wet tank collects condensate; manual drain valves at each reservoir must be opened periodically to remove water and oil.
Compressor build-up time is a diagnostic: slow build-up indicates worn compressor, leaking lines, or a stuck-open unloader.
The check valve between the compressor and the wet tank prevents backflow of air from the reservoir when the compressor stops.
Hydraulic Brake Systems
Pascal's law: pressure applied to a confined fluid is transmitted equally in all directions, so equal pressure acts on all wheel cylinder pistons.
Hydraulic advantage: force = pressure x piston area; a larger wheel cylinder piston produces greater braking force for the same line pressure.
DOT 3 and DOT 4 glycol-based fluids are hygroscopic; absorbed moisture lowers boiling point and promotes corrosion, so fluid should be replaced per interval.
DOT 5 silicone fluid is not compatible with glycol fluids and must not be mixed; mixing causes swelling of seals and loss of braking.
A residual pressure check valve in the master cylinder maintains slight pressure to drum wheel cylinders to keep cup seals seated.
A metering (hold-off) valve delays front disc application until rear drum brakes begin to apply, improving balance during light stops.
A proportioning valve reduces rear hydraulic pressure to prevent rear wheel lockup during hard braking.
Bench bleeding the master cylinder removes trapped air before installation; air in the system causes a low, spongy pedal.
A brake pedal that slowly sinks under constant pressure indicates an internal master cylinder bypass (leaking cup seal).
Mechanical and Parking Brake Systems
The parking brake on a transit bus is typically spring-applied and air-released; loss of air pressure applies the parking brake automatically.
A spring brake must be caged (manually compressed) before removal or when the vehicle must be moved with no air pressure.
The caging bolt is a maintenance tool only; it must be removed before returning the vehicle to service.
Automatic slack adjusters adjust during the return stroke when the pushrod stroke exceeds the internal adjustment threshold.
Manual slack adjusters are set by turning the adjusting hex until the lining contacts the drum, then backing off to the specified clearance.
Pushrod stroke limits are chamber-specific; exceeding the limit requires inspection of the slack adjuster, foundation brake, and chamber.
A parking brake that will not release with air applied indicates a faulty release valve, stuck relay valve, or seized spring brake chamber.
Brake lining thickness is measured at the rivet or bond line; worn linings increase pushrod stroke and reduce braking force.
Driveline parking brakes (where used) act on the driveshaft and must be inspected for output shaft and yoke wear.
Anti-Lock Braking Systems (ABS) and Electronic Controls
ABS modulates brake pressure at each wheel to prevent wheel lockup and maintain steering control during hard braking.
Wheel speed sensors generate an AC signal whose frequency is proportional to wheel speed; a damaged tone ring or air gap fault sets a code.
The ABS ECU monitors wheel speed sensors, modulator valves, and the relay valve; a fault illuminates the ABS warning lamp and may disable ABS.
The ABS modulator valve is a solenoid-controlled valve that can hold, release, or reapply pressure to a brake chamber independently of the driver's foot valve.
The ABS relay valve supplies reservoir air to the modulator valves and is controlled by the ECU.
A wheel speed sensor air gap that is too large produces a weak signal; a gap that is too small risks contact and sensor damage.
Blink-code diagnostics on older ABS systems count flashes of the ABS lamp to identify stored fault codes.
After any ABS repair, the system must be cleared of codes and the lamp must extinguish; a lamp that stays on indicates an unresolved fault.
ABS does not shorten stopping distance on all surfaces; on loose gravel, wheel lockup can build a wedge that shortens stops.
Diagnosis and Troubleshooting
A low, spongy hydraulic pedal indicates air in the system or a faulty master cylinder; a hard pedal indicates a booster or check valve fault.
A brake pull to one side indicates a dragging brake, contaminated lining, or a restricted hydraulic line on the opposite side.
Brake drag is diagnosed by comparing hub temperatures after a road test; a hotter wheel indicates a dragging brake.
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.
A compressor that runs continuously with rising pressure indicates a stuck unloader or faulty governor; a compressor that never builds pressure indicates a failed compressor or open line.
A single-circuit air brake failure should be confirmed by observing which reservoir loses pressure and which brakes remain functional.
Vibration or pulsation during braking indicates rotor thickness variation, drum out-of-round, or uneven torque on wheel fasteners.
A brake warning lamp that illuminates with the parking brake released indicates low fluid level, low air pressure, or a switch fault.
Noise on brake application can come from glazed linings, worn hardware, or a dry backing plate; isolating the wheel locates the source.
Repair Procedures and Adjustments
Brake linings and pads are replaced in axle sets; mixing lining types on the same axle can cause uneven braking.
Rotors are measured for thickness and runout; machining is only permitted if the finished thickness stays above the minimum specification.
Drum brakes are adjusted to a specified lining-to-drum clearance, then the pushrod stroke is verified at full application.
Automatic slack adjusters must be checked for correct installation and free play; a worn or improperly installed adjuster will not self-adjust.
Brake chamber pushrods must be replaced when bent, and the clevis pin and cotter pin must be properly installed.
Torque wheel fasteners to specification in a star pattern to avoid rotor distortion and uneven clamping.
Hydraulic systems are bled in the manufacturer's specified sequence, usually starting at the wheel farthest from the master cylinder.
After brake work, a road test at low speed verifies pedal feel, brake balance, and that the ABS lamp extinguishes.
Spring brake chambers must be caged and the air system drained before removal; the caging bolt must be removed before service.