Air Brake System Components and Warning Devices
Air Brake System Components and Warning Devices
This domain tests whether you can identify the parts that make compressed air do the braking and the warning thresholds that tell you the system is failing. The NC air-brake endorsement questions often hinge on exact pressures, not just component names.
Core rules
- The compressor builds pressure; the governor controls it. Cut-out is about 125 psi, cut-in about 100 psi. If pressure keeps rising past cut-out, suspect a faulty governor or unloader.
- A safety valve protects the system, usually opening near 150 psi. It is not a normal pressure regulator.
- Dual air circuits split service brakes into primary and secondary systems so one leak does not remove all braking.
- The low-pressure warning (buzzer/light) must activate before pressure drops below 60 psi. That is a "before," not "at."
- Spring brakes begin to apply when pressure falls into the 20-45 psi range. They are the emergency/parking brake, held off by air pressure.
- Dash gauges show primary and secondary reservoir pressure; the application-pressure gauge shows how much air you are applying, not reservoir supply.
Worked example
A driver shuts the engine off and fans the brake pedal. The warning buzzer sounds at 57 psi and the spring brakes set at 34 psi. Both readings are acceptable: the warning came on before 60 psi, and spring-brake application fell inside 20-45 psi. If the buzzer first sounded at 62 psi, the warning device would be late and the vehicle should be placed out of service.
Traps
Do not mix up governor cut-in with cut-out. Do not treat the safety valve as the governor. Do not expect the warning at exactly 60 psi or spring brakes at exactly 45 psi. Do not assume a gauge reading proves both circuits are healthy.