Pre-Trip Leak Test on a Tractor-Trailer
A driver is doing a pre-trip on a 2016 three-axle tractor and a tandem-axle van trailer at a Saskatoon yard at +5°C. After building to 120 psi and shutting down, the driver releases the parking brake, waits one minute, and records a drop from 120 to 116 psi over the next minute. With the service brakes fully applied and held, the gauge drops from 118 to 112 psi in one minute. The tractor protection valve is open and the trailer is charged. The low-pressure warning buzzer sounds at 58 psi, and the tractor spring brakes apply at about 42 psi. The compressor build-up from 85 to 100 psi takes 50 seconds at fast idle. The driver notes that the trailer gauge drops 3 psi/min when the trailer is isolated, while the tractor gauge drops 1 psi/min. The company standard is 2 psi/min released and 3 psi/min applied for the combination.
1. Interpret each test result against the company standard and identify the failing circuit.
Read the worked answer
Released leak: 120 to 116 psi is 4 psi/min, which exceeds the 2 psi/min company limit, so the combination fails the released test. Applied leak: 118 to 112 psi is 6 psi/min, which exceeds the 3 psi/min applied limit, so the combination also fails the applied test. Build-up: 50 seconds exceeds the 45-second target, so the compressor is marginal or there is a delivery restriction. The isolated results help locate the fault: the trailer at 3 psi/min exceeds the 2 psi/min released limit, while the tractor at 1 psi/min is within it. The combination leak is therefore mostly in the trailer circuit, although the tractor still needs to be checked because 1 psi/min is not zero. The low-pressure warning at 58 psi is acceptable because it must occur before 60 psi, and the spring brakes applying at about 42 psi is within the normal 40-45 psi range. Those two passes do not offset the leaks; every test must pass before the unit is dispatched.
2. What is the correct isolation procedure to confirm whether the leak is in the tractor or trailer, and what does the 3 psi/min trailer result tell you?
Read the worked answer
Charge the system to governor cut-out, shut the engine off, chock the wheels, and release the parking brake. Record the gauge, then close the tractor protection valve or the trailer supply shut-off valve to isolate the trailer. Watch the tractor gauge for at least one minute. If the tractor gauge continues to fall, the leak is on the tractor side. Next, recharge the trailer, close the trailer supply valve, and watch the trailer gauge; if it falls, the leak is in the trailer circuit. The trailer-only result of 3 psi/min released exceeds the 2 psi/min limit, so the trailer has a real leak. It could be in a service line, a relay valve, a brake chamber diaphragm, a fitting, or a quick-release valve. Soap-test accessible joints and listen at the relay valve and chambers. Because the combination applied leak was 6 psi/min, also perform an applied test on the trailer alone, since a service-side leak may only show when the brakes are applied.
3. Is the low-pressure warning and spring brake application acceptable? Explain why the build-up result still matters even though it is only 5 seconds over.
Read the worked answer
The warning at 58 psi is acceptable because the warning must operate before the pressure falls to 60 psi. The spring brakes applying at about 42 psi is also typical and acceptable, since they should apply in the 40-45 psi range before pressure reaches zero. The build-up result still matters because 50 seconds exceeds the 45-second target and shows the compressor is not restoring pressure as fast as it should. Five seconds may look minor, but it is an out-of-specification result, and with an existing leak the margin will get worse. In cold weather, a marginal compressor plus a leak can leave the driver without enough reserve to make a safe stop. The correct decision is to repair the leaks, recheck compressor performance, and repeat the full pre-trip test sequence before the vehicle goes into service.