Pre-Trip Inspection of Air Brake Systems
This domain tests whether you can find an air brake defect before the wheels roll. A defect you miss during the walk-around is scored against you, so know the order and the acceptance limits.
Compressor and air supply
- Check the compressor drive belt and pulley for cracks, glazing and correct tension.
- Look and listen for leaks at hoses, lines and fittings along the frame and at each wheel.
- Drain the air tanks daily to remove moisture and oil, and know where each drain valve is.
Brakes at each wheel
- Check the slack adjuster with the parking brake released and the wheels chocked: pull it by hand. Movement of more than about 1 inch where it attaches to the pushrod means it needs adjustment.
- With the brakes applied at 90-100 psi, measure pushrod travel against the chamber's limit; a long stroke means an out-of-service brake.
- Inspect brake chambers, pushrod, clevis and cotter pin, check linings for cracks, oil, or missing and loose parts, and check drums for cracks and heat damage.
Warnings and function tests
Build pressure, shut the engine off, and fan the brakes: the low pressure light and buzzer must come on before 60 psi. Check the build rate from 85 to 100 psi at fast idle, then test the parking brake, a service brake stop from about 5 mph, and the brake lights.
Worked example
A slack adjuster moves 1 1/2 inches by hand. The limit is about 1 inch, so the brake is out of adjustment. A manual adjuster must be set and the pushrod stroke re-checked before the truck leaves the yard.
Traps
There is no brake fluid to check on an air brake vehicle, tanks are drained daily rather than yearly, and the air tests are performed with pressure in the system, not with empty tanks.