Air Brake System Components and Operation
What the exam tests
This domain tests whether you can trace compressed air from the compressor to the brake chamber and explain what each component does during application and release. Ontario Z-endorsement questions use standard air-brake terminology, so learn component names and functions precisely.
Core rules and concepts
- The engine-driven compressor supplies air. The governor controls cut-out, commonly 120–135 psi, and cut-in, usually 20–25 psi lower, by loading and unloading the compressor.
- Reservoirs store air. A safety valve protects against overpressure, normally opening near 150 psi; drain cocks remove moisture and oil.
- The foot valve, or treadle valve, meters air pressure. Relay valves speed application on long wheelbases; quick-release valves speed release.
- A brake chamber converts air pressure into pushrod travel. The slack adjuster converts that travel into S-cam rotation, forcing the brake shoes against the drum.
- Spring brakes are applied by mechanical springs when air pressure is lost. Air pressure holds them off; they are not air-applied.
- The tractor protection valve and glad hands separate tractor and trailer air lines. The emergency/supply line charges the trailer; the service/control line signals brake application.
Worked example
A tractor gauge rises to 125 psi and holds while the compressor unloads. At about 100 psi the compressor loads again. That is normal governor operation. If the gauge never reaches cut-out and the compressor runs continuously, check the governor signal line, compressor output and system leakage before blaming the gauge. If the safety valve releases near 150 psi, suspect a failed governor or a restricted signal line.
Traps that cost marks
Mixing up supply and control lines; saying spring brakes are air-applied; ignoring the relay valve’s role; and treating the governor as a brake-pressure regulator. The governor controls compressor loading, not braking force.