Air Brake System Components and Functions
Domain 1 covers the parts that generate, store, control, and apply air pressure. Georgia's CDL Air Brakes Test asks you to identify each component and explain what it does, including how pressure is regulated and how a pedal movement becomes braking force at the wheel.
Core components and flow
- An engine-driven compressor builds pressure. The governor cuts the compressor out at about 120-135 psi and back in at about 100 psi. An air dryer removes moisture; a safety valve protects the tank at about 150 psi; check valves prevent backflow; manual drain valves at the tanks, especially the wet tank, remove water.
- Tanks store air. The foot or treadle valve meters air to a relay valve; the relay valve applies the rear brakes faster; a quick-release valve speeds release.
- A brake chamber converts air pressure into pushrod force. The slack adjuster converts pushrod stroke into S-cam rotation. The foundation brake, with S-cam, shoes, and drum, creates friction. A spring brake chamber has a mechanical spring for parking and emergency use; air pressure releases it.
- The supply gauge shows tank pressure; the application gauge shows delivery pressure. The low-pressure warning light or buzzer must activate before 60 psi. The stop light switch may be air or electric.
Worked example
A truck builds to 130 psi, the compressor stops, and pressure falls to 105 before it restarts. That is normal governor cut-out and cut-in. If pressure keeps rising to 150 and the safety valve vents, the governor is not controlling the compressor; the safety valve is doing its backup job. If pressure rises very slowly and the gauge drops when parked, suspect a leak or weak compressor, not the safety valve.
Traps that cost marks
Do not confuse supply pressure with application pressure. Spring brakes apply by spring force when air is lost; they do not apply by adding air. The safety valve is not the normal pressure regulator. The wet tank is the first tank after the compressor and needs draining. A slack adjuster is not a brake chamber.
Key points
- Compressor builds air; governor controls cut-in and cut-out.
- Safety valve protects the tank at about 150 psi.
- Treadle valve controls service brake application.
- Spring brakes apply mechanically when air pressure is lost.
- Supply gauge and application gauge show different pressures.