Air Brake System Components and Operation
This domain tests how an air brake system produces, stores, controls, and applies compressed air. Expect questions on the compressor and governor, storage tanks and valves, brake chambers, slack adjusters, foundation brakes, and the dual-circuit design.
Core rules and components
- The engine-driven compressor supplies air. The governor controls compressor operation: it cuts the compressor out at about 125 psi and cuts it back in at about 100 psi. These are control points, not the system’s pressure limit.
- The safety valve protects the tanks, usually opening near 150 psi. A check valve allows air to flow in one direction only, preventing backflow if pressure drops.
- The wet tank receives air first and collects moisture and oil. Primary and secondary tanks supply the two brake circuits. Drain tanks daily; an air dryer removes moisture before it reaches the tanks.
- The treadle valve controls service brake application. The tractor protection valve protects the tractor air supply if the trailer separates or its lines fail.
- A brake chamber converts air pressure into push-rod movement. The push rod moves the slack adjuster, which rotates the S-cam or wedge to expand the brake shoes. Air disc brakes use a similar chamber-and-caliper action.
- Spring brakes are parking and emergency brakes. Air pressure releases them; loss of air lets the springs apply them.
Worked example
A driver watches the gauge rise to 125 psi and the compressor stop. After several brake applications the gauge falls to 100 psi and the compressor restarts. Reasoning: 125 psi is governor cut-out; 100 psi is cut-in. If the gauge kept falling below 100 psi with the engine running, the fault would be in the governor, compressor, or unloader-not in the safety valve.
Traps that cost marks
Do not confuse the governor cut-out with the safety-valve setting. Do not call spring brakes “service brakes.” Do not assume the wet tank is just extra capacity; it is the moisture-collection point. Do not overlook check valves when diagnosing air flowing the wrong way.
Key points
- Governor cut-out about 125 psi; cut-in about 100 psi.
- Safety valve about 150 psi; check valves prevent backflow.
- Wet tank first; drain tanks daily; air dryer removes moisture.
- Brake chamber push rod moves the slack adjuster and S-cam or wedge.
- Spring brakes apply by spring force when air pressure is lost.