Air Brakes and Brake System Checks on Combination Vehicles
This domain tests your ability to inspect and test the tractor-trailer air brake system, including trailer protection devices and leakage limits. You must know the order of tests and what a failed reading means.
Core rules and checks
- Drain air tanks and check for moisture; inspect lines, glad hands, and seals for leaks or damage.
- Build system pressure to governor cut-out (about 120-125 psi). With the engine off and brakes released, watch the gauge: leakage should not exceed 3 psi per minute for a single vehicle or 4 psi for a combination.
- Apply the brakes fully and hold. Leakage should not exceed 4 psi per minute for a single vehicle or 6 psi for a combination.
- Check the low-pressure warning: the buzzer and light must activate before pressure falls below 60 psi.
- Continue to watch pressure; the spring brakes should apply at about 20-45 psi.
- Check the tractor protection valve: it should close and protect the tractor system if trailer air pressure is lost.
- Test trailer brakes separately with the trailer hand valve or tractor parking brake control, and confirm the trailer spring brakes hold.
- With ABS, know the dash lamp sequence; a fault lamp means the system needs attention.
Worked example
After charging to cut-out and shutting down the engine, a driver releases all brakes and sees the gauge fall from 125 to 119 psi in one minute. That is 6 psi, exceeding the 4 psi combination limit. The driver should not dispatch; the excessive leak could reduce braking performance and trigger trailer spring brakes.
Traps
Testing with the engine running, skipping the applied leak test, ignoring a slow gauge drop, failing to check trailer brakes separately, and assuming a low-pressure warning is only a light.