Air Brakes and Combination Vehicle Braking Systems
The system on a combination
The compressor charges the reservoirs; the governor cuts out near 125 psi and cuts in near 100 psi. A safety valve protects the tanks at about 150 psi. The low-pressure warning (buzzer and light) must activate by 60 psi, and spring brakes apply somewhere between 20 and 45 psi. On a combination, the tractor protection valve closes in that same 20–45 psi range, trapping tractor air so you can still stop the power unit if a trailer line breaks.
Knowing the valves
The red trailer air supply knob charges the trailer when pushed in, and shuts off and sets the trailer spring brakes when pulled out. The yellow diamond is the parking brake valve. Mixing them up is a classic failure - pulling red instead of yellow dumps the trailer brakes and can leave you dragging a locked trailer.
Leak tests
With the system fully charged and the engine off, release the brakes and watch for one minute: loss should not exceed 3 psi on a single vehicle or 4 psi on a combination. Then hold the foot valve applied and confirm no more than 4 psi per minute. Finally, fan the brakes down to check that the warning sounds by 60 psi and the spring brakes set.
Worked example
Gauge reads 120 psi; one minute later it reads 116 psi. Four psi of loss is at the limit for a combination - acceptable only if the applied test and the low-pressure warning check also pass. A 6 psi drop is a defect: do not dispatch.
Traps
Testing leakage with the engine running; skipping the applied test; ignoring a slow leak because the gauge "still looks fine"; pulling the red knob to park.