Air Brake Safety Practices and Test Procedures
This domain covers the practical tests and habits that prove your brakes will hold before you need them, plus parking practice that keeps the system working.
Pull (tug) test
Apply the parking brake, put the transmission in the lowest gear, release the clutch gently and try to pull. The vehicle should not move. If it does, the spring brakes are not holding and the vehicle must not be dispatched.
Five mph service brake test
In low gear at about 5 mph, apply the brakes firmly. The vehicle should stop straight without pulling to one side; a pull points to a sticking brake, contaminated lining or bad adjustment. Do this after any brake work.
Brake check before a grade
At the top of a downgrade, stop and check the service brakes with an applied leak test and a firm pedal, and confirm the spring brakes hold. A tug test alone proves only the parking brake, not the brakes you will ride going down.
Parking and chocking
Set the parking brake, put the transmission in the lowest gear, and chock the wheels on a grade or whenever the vehicle is left running. Turn the front wheels so the vehicle would roll into the curb or off the road, not into traffic.
Heat and cold
Do not set the parking brake right after a hard stop; hot shoes can stick to the drum. Cool down first or park with the wheels chocked. Drain tanks daily, especially in freezing weather.
Worked example
A driver at the top of a long grade does a tug test, sees the truck hold, and starts down. Halfway down the brakes fade. The tug test only checked the spring brakes; the service brakes were never checked. The right call was an applied leak test and a firm-pedal check before descending.
Traps
- Testing the service brakes at highway speed.
- Leaving the vehicle with no chocks and the engine running.
- Setting the parking brake on hot drums.
- Driving with the yellow knob out, dragging the spring brakes.