Air Brake System Components and Functions
This domain tests whether you can identify the parts of a Kansas CDL vehicle’s air brake system, state what each part does, and connect a symptom to the component responsible. Expect questions that describe a pressure behavior or a failed part and ask for the cause.
Core components
- Compressor: engine-driven; produces compressed air. It does not apply the brakes by itself.
- Governor: senses reservoir pressure and controls the compressor. It cuts the compressor in at about 100 psi and out at about 120–135 psi.
- Reservoirs: store air; drain valves remove water and oil.
- Check valves: allow air to flow one way only, protecting against backflow and loss through a failed circuit.
- Safety valve: usually set at 150 psi; protects tanks from overpressure.
- Treadle (foot) valve: the driver’s service brake control; sends air to the service chambers.
- Service brake chamber: converts air pressure into pushrod motion. The spring chamber is separate and applies mechanically.
- Slack adjuster: links the pushrod to the camshaft; poor adjustment wastes pushrod travel.
Worked example
A tractor gauge rises to 135 psi, the compressor stops, pressure falls to about 105 psi, and the compressor restarts. This is normal governor cut-out and cut-in. If pressure instead fell from 135 psi to 80 psi before restart, suspect a leak, a faulty governor, or a compressor problem.
Traps
- Saying the compressor “holds the brakes on.” Air pressure releases spring brakes.
- Confusing the safety valve (tank overpressure) with the low-pressure warning (driver alert).
- Forgetting that check valves, not the governor, isolate circuits.
Key points
- The compressor makes air; the governor controls when it pumps.
- Normal cut-out is about 120–135 psi; cut-in is about 100 psi.
- The safety valve protects tanks at about 150 psi.
- Service chambers apply brakes with air; spring chambers apply mechanically.
- Check valves prevent backflow and circuit loss.