Air Brake System Components and Function
Air Brake System Components and Function
This domain tests how an air brake system creates, stores, controls, and applies compressed air. Expect questions on the compressor and governor, reservoirs, valves, brake chambers, slack adjusters, and spring brakes-not just names, but what each part does when you press the treadle.
Core rules and concepts
- The engine-driven compressor supplies air. The governor controls compressor cycling: it cuts the compressor out near 125 psi and back in around 100 psi, so normal operating pressure stays roughly 100–125 psi.
- The wet tank receives air first and collects moisture; primary and secondary tanks supply separate brake circuits. Check valves stop backflow, the safety valve protects against overpressure, and drain valves remove water and oil.
- The foot valve (application valve) meters air to the brake chambers. In a dual-circuit system, one circuit can still provide braking if the other fails.
- A brake chamber converts air pressure into pushrod stroke. The slack adjuster turns that stroke into rotation of the foundation brake camshaft (or applies a disc caliper). Too much pushrod travel means inadequate braking.
- Spring brakes are applied by mechanical springs and released by air pressure. Loss of air pressure therefore causes them to apply. The parking control is typically the yellow diamond knob; the trailer supply is the red octagon.
Worked example
A tractor’s gauge climbs to about 125 psi, the compressor stops, and pressure later falls to about 100 psi before the compressor restarts. That is normal governor cut-out/cut-in behavior. If the compressor ran constantly but pressure stayed at 60 psi, the fault is not the governor alone; look for a major leak, failed compressor, or closed valve.
Traps
Do not confuse the wet tank with a service tank, or the safety valve with the low-pressure warning switch. Do not say spring brakes are held off by springs; air holds them off.