Air Brake System Components and Operation
Domain 1, Air Brake System Components and Operation, tests whether you can trace compressed air from the compressor to the brake chamber and explain what happens when a component fails. Colorado's CDL air brake test leans on the federal model, so expect questions about governors, tanks, valves, chambers, and gauges.
The air path
The engine-driven compressor sends air through a one-way check valve into the supply (wet) tank. An air dryer or alcohol evaporator removes moisture before the air reaches the primary and secondary tanks. The foot (treadle) valve meters air to the relay valve, which sends pressure to the service brake chambers. Each chamber's push rod moves a slack adjuster, which twists the S-cam (drum brake) or applies the disc brake.
Key settings and parts
- Governor: starts the compressor at about 100 psi (cut-in) and stops it at about 125 psi (cut-out).
- Safety relief valve: usually set to open at 150 psi; it protects the system if the governor fails.
- Low-pressure warning: buzzer and light must activate before pressure falls below 60 psi.
- Spring brakes: parking and emergency brakes; they apply automatically at roughly 20-45 psi and release only with enough air pressure.
- Supply gauge: shows tank pressure. Application gauge: shows pressure you are applying to the brakes.
- Drain valves: remove water and oil from tanks; drain daily.
Worked example
A driver sees the compressor cut out at 125 psi, then cut in at 100 psi. Later the dash gauge climbs past 130 psi and the safety valve releases near 150 psi. Reasoning: the governor has failed to stop the compressor, so the safety valve is doing its backup job. The driver should stop, shut down, and get the governor repaired; driving with a failed governor risks an overpressure failure.
Traps that cost marks
Do not confuse the safety valve with the governor: the governor controls normal cut-out; the safety valve is the last-resort relief. The application gauge is not the supply gauge. A spring brake is not merely a parking brake-it is also the automatic emergency brake. Finally, draining tanks is not optional: water and oil damage valves and can cause brake drag or failure.