Air Brake System Components and Function
This domain tests whether you can identify each part of a CDL air brake system and explain what it does - especially how the compressor, governor, reservoirs, valves, brake chambers, and spring brakes work together to stop the vehicle. Expect questions on gauge readings, cut-in/cut-out pressures, and the difference between service and parking brakes.
The pressure path
Air is drawn in and compressed by the engine-driven compressor, then stored in reservoirs (wet tank first, then dry tanks). A governor watches reservoir pressure and controls the compressor: it cuts out at roughly 125 psi and cuts back in around 100 psi. A safety valve on the reservoir opens near 150 psi if the governor fails. A check valve keeps air from flowing backward if the compressor fails.
The foot valve (treadle) sends air to the brake chambers through relay valves; the push rod moves the slack adjuster, which rotates the S-cam to expand the shoes against the drum. Quick-release valves speed up release at the wheels.
Dual circuits and spring brakes
Dual systems split the vehicle into two independent circuits; a failure in one should still leave partial braking. Spring (parking) brakes are applied by heavy mechanical springs and released by air pressure; if pressure falls to roughly 20–45 psi, they apply automatically. The yellow push-pull knob controls them.
Gauges
The supply gauge shows reservoir pressure; the application gauge shows pressure you are applying with the foot valve.
Worked example
A driver at idle sees 120 psi. After several stops the gauge falls to 100 and the compressor runs until 125, then stops. Is this normal? Yes: 100 psi is the governor cut-in and 125 psi is cut-out. A gauge that keeps climbing past 135–150 would point to a governor or safety-valve problem.
Traps
Do not confuse the supply gauge with the application gauge, the safety valve with the governor, or spring brakes with hydraulic brakes. The trailer hand valve is not a parking brake.