Air Brakes and Braking Systems on Combination Vehicles
Air Brakes and Braking Systems on Combination Vehicles tests the plumbing and the checks: which line does what, what protects the tractor when a trailer line fails, and what the numbers in the leak and low-pressure tests must be.
Layout you must know
- The dual air system has separate supply and service circuits so a failure in one does not empty the other.
- The compressor is governed to cut out around 125 psi and cut in around 100 psi; a safety valve protects the tanks near 150 psi.
- Red line = emergency (supply). Blue line = service. Color-match the gladhands.
- The emergency line charges the trailer reservoirs and holds off the trailer spring brakes. The service line applies the trailer brakes.
- The tractor protection valve closes when pressure falls too low or when the red trailer air supply knob is pulled out. It protects the tractor's air supply; the trailer's spring brakes then apply.
- Trailer spring brakes apply automatically when the emergency line loses pressure - that is the breakaway protection.
The checks examiners ask about
- Static leak test: engine off, system fully charged, brakes released - no more than 3 psi loss per minute for a single vehicle, 4 psi for a combination. Repeat with the brakes fully applied; the same limits apply.
- The low-pressure warning must sound or light before pressure drops below 60 psi.
- Spring brakes should set somewhere between 20 and 45 psi, and you generally need about 100 psi to release them.
- Brake adjustment: a pushrod stroke beyond the limit means that brake is out of adjustment; enough of them and the vehicle is out of service.
Worked example
A driver runs the leak test on a loaded combination: 120 psi, engine off, brakes released. One minute later the gauge reads 115 psi. That is a 5 psi drop against a 4 psi limit, so it fails. The likely suspects are a gladhand seal, a chafed service line, or a brake chamber. Driving on it risks losing enough pressure mid-trip to set the trailer spring brakes, which can lock the trailer wheels and jackknife the rig.
Traps
- Swapping red and blue.
- Using the trailer hand valve ('johnny bar') to hold at a light or to park. It is a test device only.
- Thinking the tractor protection valve protects the trailer's air.
- Skipping the brakes-applied half of the leak test.