Air Brakes and Brake Systems on Combination Vehicles
Air brakes on a combination - what is tested
Combination questions focus on the parts a rigid truck does not have: the tractor protection valve, the trailer reservoir and relay valve, and the two trailer lines. Expect leak-rate numbers, warning pressures, and fail-safe behavior.
The system
- The compressor fills the wet tank, then the primary (rear brake) and secondary (front brake) tanks.
- The governor cuts the compressor out around 125 psi and back in around 100 psi.
- The tractor protection valve is combination-only. If pressure falls to roughly 20-45 psi, it closes so the tractor keeps its air instead of losing it through a broken trailer line.
- The trailer carries its own reservoir and relay valve. The emergency (supply) line charges the trailer; the service line signals the trailer brakes.
- Spring brakes apply when air is lost and cannot be released without air; the caging bolt is only for moving a disabled vehicle.
The three checks
- Leakage, engine off at 90 psi: released, no more than 3 psi/min; applied, no more than 4 psi/min.
- Low-air warning: buzzer or light before 60 psi; spring brakes set between 20 and 45 psi.
- Compressor build-up: 85 to 100 psi within 45 seconds at high idle.
Worked example
A trailer's emergency line is cut and the trailer brakes lock up. Reason: the supply line pressurizes the trailer reservoir and holds off its spring brakes. Lose that pressure and the trailer brakes apply automatically - a fail-safe. A cut service line is different: the trailer simply will not brake from the foot valve, while the emergency function still works. Knowing which line does what is the tested point.
Traps
- Draining only one tank instead of all of them daily.
- Thinking the tractor protection valve protects the trailer.
- Using the trailer supply valve as a parking brake.
- Believing ABS lets you brake later or pump the pedal.