Air Brakes and Brake Systems on Combination Vehicles
How the system works
A combination vehicle uses a dual air-brake system with a tractor protection valve that separates the tractor's air supply from the trailer's. The trailer hand valve (also called the trolley valve) lets you apply only the trailer brakes; it is used during coupling checks and for testing. The trailer air supply valve (red button) controls air to the trailer, and the parking brake valve (yellow button) controls the tractor's spring brakes.
Key checks
- Static leak test: with the engine off and the system charged, release the parking brakes and watch the gauge. A loss of more than 3 psi in one minute (single vehicle) or 4 psi (combination) indicates a leak.
- Applied leak test: fully apply the foot brake and hold; loss should not exceed 4 psi in one minute.
- Low-air warning: the warning buzzer/light must activate before the pressure drops below 60 psi.
- Spring brakes: they must engage automatically when pressure falls to roughly 20–45 psi.
Traps
Questions frequently confuse the red and yellow buttons. Remember: red = trailer air supply, yellow = tractor parking brake. Another trap is the trailer hand valve - it must never be used for parking; it can leak down and release the trailer brakes.
Rule of thumb
If the low-air warning sounds while driving, stop safely and do not continue - you are minutes from losing your brakes.