Air Brake Pre-Trip: Three Failures and Twenty Minutes
Cold start, 40 degrees F. Your combination air system builds from 85 to 100 psi in 70 seconds. The governor cuts out at 122 psi and cuts back in at 100 psi. With the engine off and the system fully charged, pressure drops 4 psi in one minute with the brakes released and 5 psi in one minute with the service brake applied and held. The low-pressure warning buzzer sounds at 58 psi, and the spring brakes set with a noticeable jolt at about 35 psi. At the rear of the tractor you can hear a slow leak at a trailer gladhand coupler. Dispatch wants you rolling in 20 minutes with a 400-mile run.
1. Sort these readings into pass and fail, and quantify each one.
Read the worked answer
Passes:
- Governor cut-out at 122 psi and cut-in at 100 psi - a normal operating band with adequate spread.
- Low-pressure warning at 58 psi - the warning must occur before pressure falls below roughly 60 psi, so this is inside spec.
- Spring brakes applying at about 35 psi - the expected range is roughly 20 to 45 psi.
Failures:
- Build-up 85 to 100 psi took 70 seconds against a 45-second standard for a dual-air-system vehicle. The compressor is not keeping up, which means every brake application on the road starts you further behind.
- Static leakage 4 psi/minute against a 3 psi/minute limit for a combination vehicle. Over the limit.
- Applied leakage 5 psi/minute against a 4 psi/minute limit for a combination vehicle. Over the limit.
Three failures. Each one alone is a defect; together they describe a system that loses air faster than it makes it and cannot recover.
2. Is the audible gladhand leak already counted in the leak readings, and does that change the decision?
Read the worked answer
It is almost certainly part of the 4 psi and 5 psi figures - a leak in the charged system shows up in the static test whether or not you can hear it. So you should not add it as a fourth, separate number.
It does not change the decision, and it makes the decision easier. A leaking gladhand is not a stable defect: coupler seals that leak cold tend to leak worse hot, and a gladhand that lets go completely will drop the trailer supply line. When supply pressure falls far enough, the tractor protection valve closes and the trailer brakes apply - at best you are stopped on the shoulder, at worst you are stopped in a lane with a loaded trailer behind you and no ability to release the brakes until pressure recovers.
The measured leakage already fails, and the leak gives you a specific item to repair rather than a general complaint about the compressor.
3. What do you say to dispatch, and what goes on the report?
Read the worked answer
You do not take the load. A driver may not operate a vehicle with a defect that affects safe operation, and the pressure to move does not change the standard.
To dispatch, be specific and non-negotiable: 'Build-up 70 seconds against 45; static leak 4 against 3; applied leak 5 against 4; audible leak at the trailer gladhand. The unit is out of service until it is repaired and re-tested.' Offer options - a different tractor, a shop ETA, a re-test after repair - rather than a flat refusal, but the unit does not move loaded.
On the vehicle inspection report: record each measurement with its standard, name the gladhand as the located defect, note whether the condition appears to have existed since the last inspection, and note the time you reported it. Then re-run the full test after the repair: build-up time, static leak, applied leak, warning device, and spring brake application. Do not accept 'we tightened it' as a re-test.
If the leak was present at the last inspection and signed off as OK, that is worth telling your safety department - it is an inspection quality issue, not just a truck issue.