Air Brake System Components and Their Functions
Domain 1 tests the air brake hardware: what each component does, how air moves through the system, and why a failed part changes braking. Expect identification and function questions, not just definitions.
Core components
- Compressor: engine-driven; builds air. The governor controls cut-in (about 100 psi) and cut-out (about 125 psi).
- Reservoirs: the wet tank is first; primary and secondary tanks supply separate circuits. Check valves stop backflow; drain cocks remove water and oil.
- Safety valve: opens near 150 psi to protect tanks; it is a backup, not the normal pressure regulator.
- Treadle valve: foot valve; meters air to the service brakes.
- Slack adjusters: take up lining wear as push rods move chambers.
- Spring brakes: mechanical springs apply when air pressure is lost.
- Low-pressure warning: must activate before 60 psi.
Worked example
A driver reports pressure climbing past 130 psi and the safety valve venting. Reasoning: the governor has not cut out; the safety valve is doing its backup job. The repair is to the governor or compressor control system-never plug or disable the safety valve.
Traps that cost marks
Mixing supply and service lines; calling the wet tank a storage tank only; assuming the compressor should run constantly; treating the safety valve as the low-pressure warning.