Air Brakes and Brake System Checks on Combination Vehicles
This domain tests air-brake system checks on a combination vehicle, including leak rates, low-pressure warning, trailer protection, and trailer brake operation. A combination has two connected systems, so a tractor-only check is incomplete.
Core rules
- The tractor protection valve protects the tractor air supply if the trailer separates or pressure drops. The trailer air supply valve (red) controls air to the trailer; the trailer hand valve (yellow) is for testing trailer brakes, not parking.
- Know the warning and application points: low-pressure warning must activate by 60 psi, and spring brakes apply in the 20-45 psi range.
- With a fully charged system and engine off, release the brakes and watch the gauge for one minute. A combination should lose no more than 4 psi. Then apply the brakes; loss should not exceed 6 psi.
- Test trailer brakes separately: charge the system, apply the trailer hand valve or use the tractor protection test, and confirm the trailer brakes hold. Drain air tanks regularly.
- Never use the trailer hand valve as a parking brake.
Worked example
A combination is charged, engine off, brakes released. In one minute the gauge drops from 100 to 95 psi-5 psi. Since the combination limit is 4 psi, this fails. With brakes applied, the drop is 7 psi, also over the 6 psi limit. Reasoning: both tests exceed limits, so the vehicle is out of service until the leak is found and repaired.
Traps that cost marks
Checking only the tractor; ignoring a 5 psi drop as "close enough"; using the hand valve for parking; failing to test trailer brakes; and skipping tank drainage.