BC Air Brake Knowledge Test cheat sheet

The compressor produces compressed air; the governor senses reservoir pressure and controls when the compressor loads and unloads.

The treadle or foot valve is the driver-operated service brake control; it delivers graduated air pressure to the brake chambers.

A brake chamber converts air pressure into mechanical force, moving a push rod that applies the foundation brake.

The slack adjuster converts push-rod linear motion into rotary motion and maintains the correct brake shoe-to-drum clearance.

A relay valve speeds up rear brake apply and release by supplying reservoir air close to the brake chambers.

A spring brake chamber contains a strong mechanical spring that applies parking or emergency braking when air pressure is released.

A check valve permits airflow in one direction only, preventing air from flowing back out of a reservoir.

The tractor protection valve closes the tractor air supply to the trailer when a line separates or pressure drops, protecting tractor air.

A low-pressure warning buzzer or light alerts the driver before reservoir pressure falls below the safe operating range.

The compressor is engine-driven and builds air only when the governor places it in the loading mode.

Governor cut-out stops compressor loading at the system maximum pressure; cut-in resumes loading at a lower preset pressure.

The wet reservoir is the first tank after the compressor; it collects moisture and oil and has a drain valve.

Dry and secondary reservoirs store air for separate brake circuits; check valves prevent backflow between reservoirs.

The safety valve releases air if reservoir pressure exceeds a safe limit, such as when the governor fails to unload the compressor.

An air dryer removes moisture and oil vapor before air enters the reservoirs; it uses a desiccant cartridge and purge cycle.

Manual drain cocks must be opened to remove accumulated water and oil, then closed fully before the trip.

The system pressure gauge shows reservoir air pressure, not the amount of force applied at the brake chambers.

A typical build-up check requires pressure to rise from 85 to 100 psi within 45 seconds at fast idle.

Before driving, drain all air reservoirs and check for water or oil; close each drain fully afterward.

With the engine running, build pressure to governor cut-out and confirm the low-pressure warning activates before about 60 psi.

Static leak test: with the engine off and service brakes released, pressure should not drop more than 2 psi in one minute after stabilizing.

Applied leak test: with the engine off and service brake applied and held, pressure should not drop more than 3 psi in one minute.

With pressure below the spring-brake threshold, typically about 20 to 45 psi, spring brakes should apply automatically.

Check push-rod travel and slack adjuster free play against the manufacturer's limit; excessive stroke indicates a fault or bad adjustment.

Inspect brake chambers, hoses, lines, glad hands, and fittings for cracks, chafing, loose mounting, and air leaks.

Confirm the parking brake holds the vehicle and that a service brake application stops the vehicle without pulling to one side.

The foot valve applies service brakes progressively; greater pedal force or travel increases air pressure at the brake chambers.

Releasing the foot valve exhausts air from the service brake chambers, allowing the brakes to release.

The parking brake control applies spring brakes by exhausting air; it is separate from the normal service brake control.

The trailer hand valve applies only the trailer service brakes; it is not a parking brake and cannot hold a parked combination.

Glad hands must be clean and properly sealed; crossed or leaking lines can prevent correct trailer brake operation.

Select a lower gear before descending a grade so engine braking or a retarder can help control speed.

Snub braking means applying the service brakes firmly to reduce speed, then releasing them to cool before the next application.

Brake fade is loss of braking force from overheated drums, shoes, or pads; stop to cool and use a lower gear.

If the vehicle pulls to one side during braking, stop and inspect the brake system before continuing.

As air pressure falls below the spring-brake threshold, mechanical springs apply the brakes automatically.

The parking or emergency control can apply spring brakes; a modulated control may allow a more controlled stop.

If the trailer emergency line separates, the tractor protection valve closes and the trailer spring brakes apply.

Do not pump the brake pedal when air pressure is low; each application uses reservoir air and can speed pressure loss.

Caging a spring brake mechanically releases it for moving the vehicle, but it disables parking and emergency braking on that wheel.

If spring brakes apply at highway speed, keep steering control, move off the road, and stop safely.

If service brakes fail, use engine braking and apply the parking or emergency control progressively while steering to a safe stop.

A failed brake chamber diaphragm can cause weak braking or a dragging brake; secure the vehicle and inspect the system.

If the low-pressure warning activates, stop safely, secure the vehicle, and find the cause before continuing.

If pressure does not build, check the compressor drive, governor, air leaks, and gauge; the vehicle must not be driven until repaired.

A governor that fails to unload can cause overpressure; the safety valve releases air and the system needs repair.

Listen for escaping air and apply soapy water to fittings and hoses; bubbles indicate a leak that can cause pressure loss.

Moisture in an air line can freeze and block airflow; use the air dryer and alcohol evaporator, never an open flame.

A dragging brake can overheat, smell, smoke, and reduce engine power; stop, cool, and inspect before continuing.

If a spring brake will not release, check air pressure, the parking control, and the relay valve; cage only for a safe move.

Place wheel chocks before working under a vehicle because air pressure alone is not a reliable holding device.