Air Brakes
Air Brakes
The Air Brakes domain tests how the system makes and stores air, how it fails, and how to test it. New Mexico requires this knowledge for an air-brake-equipped CMV, and a failed air-brake check can end a road test.
How the system works
The compressor charges the tanks; the governor cuts the compressor in and out. The safety valve opens at about 150 psi. A low-air warning must activate by 60 psi, and spring brakes apply in the 20-45 psi range. Dual systems have separate circuits, so one failure should still leave some braking.
Tests
With the engine off and tanks full, release the parking brake and watch the gauge for one minute. Leakage should not exceed 3 psi for a single vehicle or 4 psi for a combination. Apply the service brake and repeat the check. Then test the low-air warning and spring-brake application, and check the safety valve.
Worked example
A tractor-trailer loses 5 psi in one minute during the static leak test. That fails because the combination limit is 4 psi per minute. Do not pass it by pumping the pedal; find the leak or place the vehicle out of service.
Using air brakes
Brake early and smoothly, and avoid fanning the pedal. On long downgrades, select a safe gear before the slope and use engine braking. With ABS, hold firm steady pressure and steer; do not pump the brakes.
Traps that cost marks
- Confusing the 150-psi safety valve with the 60-psi warning.
- Using the spring brakes as service brakes.
- Pumping ABS brakes.
- Skipping the applied-leak test.