Air Brake System Components and Operation
Components and how pressure moves
This domain tests whether you can identify air brake parts and explain how compressed air becomes braking force. The exam favors function questions: which part controls pressure, which part stores it, and which part applies the brake.
- The engine-driven compressor builds air. The governor controls when it loads and unloads; typical cut-out is about 120–125 psi and cut-in about 100 psi.
- Air passes through a check valve into the wet tank, then to the dry tank that supplies the foot valve. Drain both tanks daily.
- The safety valve on a tank protects against overpressure, usually opening near 150 psi. It is not a governor.
- The foot valve (treadle) meters air to the brake chambers. In a service brake chamber, air pressure pushes a diaphragm and pushrod; the pushrod moves the slack adjuster and S-cam or wedge to spread the shoes.
- Spring brakes are held off by air. If pressure drops, springs apply the brakes; this is why you hear a parking-brake application at low pressure.
Worked example
After a cold start, the gauge climbs from 60 to 90 psi while the compressor runs. This is normal: the governor has not yet reached cut-out. When the gauge reaches about 122 psi and the compressor unloads, the governor has cut out. If pressure keeps rising toward 150 psi and the safety valve vents, suspect a governor or unloader fault, not a normal cut-out.
Traps
Do not confuse the service brake, which uses air to apply, with the spring brake, which uses air to release. Do not call the wet tank the supply tank for the foot valve. Do not assume the safety valve regulates normal pressure.