Air Brake System Components and Function
What this domain tests
MS CDL air-brake questions in this domain ask you to identify the parts that produce, store, control, and apply air pressure, and to explain what each part does when pressure rises, falls, or is lost.
Core flow and components
- Compressor: engine-driven; pumps air into the system.
- Governor: senses reservoir pressure; cut-in is about 100 psi and cut-out about 120–125 psi. At cut-out it signals the compressor unloader to stop pumping.
- Reservoirs/tanks: store air; drain valves remove water and oil; a safety valve usually opens near 150 psi to protect the tank.
- Check valves: allow air in but prevent backflow, protecting other circuits.
- Foot/treadle valve: the driver's service-brake control; treadle movement controls application pressure.
- Brake chambers and slack adjusters: convert air pressure into pushrod stroke; slack adjusters keep stroke in adjustment.
- Spring brakes: applied by spring force when air is released; air pressure releases them. They serve as parking and emergency brakes.
- Low-pressure warning: must activate before pressure falls below 60 psi.
Worked example
A driver builds pressure to 150 psi and the tank safety valve releases air. Which component most likely failed? The governor or compressor unloader. The governor should cut out the compressor around 125 psi; if it does not, pressure keeps rising until the safety valve opens. Replacing the safety valve alone does not fix the cause.
Traps
Do not confuse cut-in with cut-out. Do not call the safety valve a pressure regulator. Do not say spring brakes are applied by air-they are released by air and applied by spring force.