Air Brakes
Air Brakes is a separate written and skills test, but the General Knowledge exam still expects you to explain how the system builds, holds, and loses pressure.
How the system works
- An engine-driven compressor charges the tanks. The governor cuts the compressor out at its set pressure (commonly in the 100–125 psi range) and cuts it back in roughly 15–20 psi lower.
- A safety valve opens at 150 psi; the low-pressure warning light and buzzer must activate before pressure falls below 60 psi.
- Spring brakes apply mechanically when pressure drops to roughly 20–45 psi. They are not just a parking brake.
The three tests
- Leak test: engine off, parking brake released, service brake applied and held. Loss must not exceed 3 psi per minute on a single vehicle or 4 psi on a combination.
- Warning test: fan off air; the warning must come on before 60 psi.
- Spring brake test: keep fanning; the brakes must apply between 20 and 45 psi.
Worked example: A combination driver builds 120 psi, shuts down, releases the parking brake, and holds the service brake. The gauge drops from 120 to 115 in one minute. That is 5 psi - over the 4 psi maximum for a combination - so the unit should be written up and not driven until the leak is fixed.
Traps: leaving the parking brake set during the leak test, which hides the real leak; pumping the pedal instead of holding it; confusing the 60 psi warning with the 20–45 psi spring brake application; fanning so fast you miss the gauge.