Air Brake System Operation in Combination Vehicles, Including Trailer Brakes and Antilock Brake Systems
This domain tests how tractor and trailer air systems share pressure, how to test them to the correct limits, and what ABS actually does.
Tractor protection valve
It isolates the tractor's air supply if a trailer line separates or the trailer breaks away, which sets the trailer's spring brakes. That is why a rig stops when the emergency line pops.
The leak tests
Build pressure to governor cut-out (about 120–125 psi), shut the engine off, and let the system stabilize.
- Static test, brakes released: loss must not exceed 3 psi per minute for a single vehicle and 4 psi per minute for a combination.
- Applied test, brakes fully applied and held: loss must not exceed 4 psi per minute for a combination.
- Low-air warning must activate by 60 psi; spring brakes should apply between 20 and 45 psi.
Trailer checks
Charge the trailer tanks, then apply the trailer brakes with the hand valve and try to pull against them in a low gear. The trailer must hold the tractor. The trailer ABS lamp should come on at power-up and then go out.
ABS
The yellow lamp signals a fault in that unit's ABS. Normal braking remains, but the wheels can lock. With ABS working, hold firm pedal pressure and steer - do not pump.
Worked example
A combination loses 3.5 psi in one minute with the brakes released. That is within the 4 psi/min combination limit, so it passes. The same 3.5 psi loss on a single vehicle would fail the 3 psi/min limit. Knowing which limit applies is the whole point of the question.
Traps
- Applying the single-vehicle leak limit to a combination.
- Assuming ABS shortens stopping distance - it preserves steering control.
- Testing trailer brakes before the trailer tanks are charged.
- Pumping the pedal with ABS.