Air Brakes: System Checks and Safe Use
Air brake system and tests
Air brakes use compressed air to apply the service brakes and spring pressure to apply the parking/emergency brakes. A dual system has two separate circuits so a leak in one does not leave you without brakes. Know the supply gauge, application gauge, low-pressure warning, and spring brake operation.
Pre-trip checks
- Drain air tanks daily to remove water and oil.
- Build pressure to governor cut-out, usually around 125 psi. The compressor should cut in near 100 psi.
- With the engine off and system fully charged, release the parking brake and watch the gauge for one minute: loss must not exceed 3 psi.
- Apply the service brake and hold it for one minute: loss must not exceed 4 psi.
- Warning must activate before pressure drops below 60 psi. Spring brakes should apply between 20 and 45 psi.
- Check slack adjuster pushrod travel and brake chamber condition; excessive travel means adjustment is needed.
Worked example
You charge the system, shut off the engine, release the parking brake, and lose 5 psi in one minute. That fails the 3 psi limit for the released-brake test. Do not drive; find and fix the leak. If the loss were 3 psi or less, the released test would pass. The applied-brake test has a separate 4 psi allowance.
Traps
- Fanning the brake pedal on a long downgrade instead of selecting a low gear.
- Riding the brakes until they overheat and fade.
- Skipping tank drains, then blaming the compressor for moisture.
- Ignoring a low-pressure warning that comes on before 60 psi.
- Assuming air brakes react instantly; brake lag still adds distance.