Air Brake System Components and Functions
What the exam tests
This domain asks you to identify air-brake parts and explain what each one does. Expect questions that link a component to its function, its normal pressure setting, or the symptom of its failure.
Core components and jobs
- Compressor: engine-driven pump that builds air. The governor controls it, typically cutting out near 125 psi and cutting in near 100 psi.
- Reservoirs: store air. Drain cocks remove water and oil; the safety valve protects the tank, usually near 150 psi.
- Check valve: one-way flow, preventing air from draining back.
- Treadle valve: driver's foot valve that meters application pressure.
- Relay valve: speeds application and release at remote chambers. Quick-release valve exhausts air quickly.
- Brake chamber: converts air pressure into pushrod force. The slack adjuster multiplies that force and transfers it to the S-cam, wedge, or disc mechanism.
- Spring brake: parking and emergency brake; it applies when air pressure is lost.
Worked example
A gauge rises to 125 psi, the compressor stops, and it restarts at 100 psi. That is normal governor operation. If the safety valve releases near 150 psi, the governor is not the part to adjust first; the system is over-pressurizing and needs diagnosis.
Traps that cost marks
Do not confuse the safety valve with the governor; the governor regulates compressor output, while the safety valve is a last-resort pressure relief. A check valve is not a drain. The spring brake is not the normal service brake. Not every chamber or valve has the same function, so match each part to its job rather than memorizing a list.