Checking Air Brakes on a Double or Triple
The sequence matters
Test the air system with the whole combination connected, because every extra glad hand, hose, and dolly is another place to leak.
- Build-up: with the engine at fast idle, pressure should rise from 85 to 100 psi within 45 seconds in a combination vehicle. Slower means a problem - do not drive.
- Static leak test: shut the engine off and let pressure stabilize. With the brakes released, the combination should lose no more than 3 psi in one minute. Hold the brake pedal fully down and it should lose no more than 4 psi in one minute. A single vehicle's limits are 2 and 3 psi.
- Low-pressure warning: the buzzer and light must come on before pressure drops below 60 psi.
- Spring brakes: they must apply when pressure falls to somewhere in the 20 to 45 psi range.
- Tractor protection valve: with the system charged and the trailer air supply valve pushed in, it should pop out and seal the lines when pressure drops, with no air leaking past it.
When charging from empty, the compressor must be able to bring the entire combination up; with more trailers the volume is larger, so a slow build-up points to a leak or a weak compressor.
Traps
Questions swap the single-vehicle and combination numbers, or say the warning should sound "at 60 psi" instead of before it. Another favorite: testing with the trailers disconnected, which hides leaks.
Rule of thumb
Combination: 85–100 psi in 45 seconds, 3 psi lost released, 4 psi applied, warning by 60, spring brakes by 20–45.