Air Brake System Components and Fundamentals
Scope
The Air Brake System Components and Fundamentals domain tests whether you can identify the parts that generate, store, and apply air pressure and explain what each part does. Expect questions on compressor and governor operation, safety valves, reservoirs and drains, check valves, the foot valve, brake chambers, slack adjusters, and foundation brakes.
Core rules
- The engine-driven compressor builds pressure. The governor controls the compressor: it typically cuts out near 120–125 psi and cuts back in near 100 psi.
- The safety valve protects the tanks from overpressure and normally opens near 150 psi; it is not the governor.
- Reservoirs store air. Drain them daily to remove water and oil. One-way check valves keep air from flowing backward.
- The treadle (foot) valve applies air progressively; more pedal pressure means more braking force.
- Air pressure pushes the brake chamber pushrod. The slack adjuster turns that stroke into S-cam or wedge action. Too much pushrod travel means the brake is out of adjustment.
- Know the difference between supply air and service air, and between a leak and normal governor cycling.
Worked example
The gauge reaches 125 psi and the compressor stops. Later it restarts at 100 psi and climbs again. That is normal governor action: cut-out near 125 psi, cut-in near 100 psi. If pressure kept rising past 150 psi, suspect a faulty governor or a blocked line; the safety valve is only a backup.
Traps
Do not confuse the governor with the safety valve. Do not assume every slack adjuster can be fixed by hand adjustment; a mechanical problem may remain. Do not diagnose normal cut-in/cut-out cycling as a leak.