Air Brake System Components and Functions
What the exam tests
This domain checks whether you can identify each part of an air brake system and explain what it does, not just name it. Expect questions that pair a component with its failure symptom or with the pressure at which it acts.
Core components and rules
- Compressor: engine-driven pump; builds and maintains air pressure. It does not store air.
- Governor: senses reservoir pressure and turns the compressor on (cut-in, about 100 psi) and off (cut-out, about 125 psi).
- Reservoirs (storage tanks): hold compressed air; drain valves at the bottom remove water and oil.
- Safety valve: in the tank; opens at about 150 psi to protect the system if the governor or compressor over-pressurizes.
- Check valve: one-way flow; keeps air from backflowing out of a tank.
- Air dryer / alcohol evaporator: removes moisture or prevents ice in cold weather.
- Foot (treadle) valve: driver's service brake control; the farther you push, the more air pressure reaches the chambers.
- Brake chamber and push rod: air pressure moves a diaphragm and push rod, which turns the slack adjuster.
- Slack adjuster: converts push-rod motion into brake shoe or disc force and adjusts for lining wear.
- Spring brakes: held off by air; apply by spring force when air is lost or the parking control is pulled.
Worked example
A truck's gauge rises to 125 psi and stops; when pressure falls to 100 psi the compressor restarts. That is normal governor cut-out and cut-in. If the gauge sits at 100 psi and the compressor never restarts, the fault is in the governor or its signal, not the safety valve. The safety valve only opens near 150 psi.
Traps that cost marks
- Confusing the safety valve (overpressure protection) with the check valve (one-way flow).
- Saying the compressor "stores" air; the reservoirs store it.
- Treating spring brakes as normal service brakes.
- Forgetting that drain valves remove moisture, not pressure.