Pre-Trip Leak Test After a Compressor Replacement
You are doing a pre-trip on a three-axle tractor with duals pulling a tandem-axle van. The combination was just serviced for a compressor replacement. With the system fully charged, engine off, and parking brakes released, pressure drops from 105 to 103 psi in one minute. With the service brake fully applied and engine off, pressure drops from 105 to 100 psi in one minute. With the engine at fast idle, buildup from 85 to 100 psi takes 70 seconds. The low-pressure warning buzzer comes on at 50 psi. The governor cuts out at 130 psi and cuts in at 100 psi. Your dispatcher wants the truck on a 400-mile run in 30 minutes. The shop says the leak is just the new compressor seating in.
1. Evaluate each test result against the standard for a combination vehicle and identify which failures are out-of-service.
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Use the combination-vehicle limits, not the single-vehicle limits. With brakes released, a drop of 2 psi in one minute passes: the limit is 3 psi for a combination. With the service brake applied, a drop of 5 psi in one minute fails: the limit is 4 psi for a combination. Buildup from 85 to 100 psi in 70 seconds fails the 45-second standard. The warning buzzer at 50 psi also fails: it must come on before pressure drops below 60 psi. The governor cut-out at 130 and cut-in at 100 are not the decisive defects here. Three separate failures - applied leakage, slow buildup, and a late warning - make the vehicle out of service. The 2 psi unapplied drop does not cancel them.
2. The shop claims the compressor is seating in. How would you reason about that claim, and what would you do before moving the vehicle?
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A new compressor can cause some initial gauge movement, but it does not explain a 5 psi drop in one minute with the service brake applied, a 70-second buildup, and a warning at 50 psi. Those are separate performance failures, and the applied leakage test is specifically designed to reveal a leak in the service-brake circuit that may not show with the brakes released. Seating in is not a repair order. Before moving, I would document the readings, red-tag the vehicle, and turn it over to a qualified technician for repair and a complete re-test. I would not accept a road test or a dispatcher's schedule as evidence that the system is safe. If the shop wants to re-test after repairs, I would want to see all four results pass: unapplied leakage, applied leakage, buildup time, and warning activation before 60 psi.
3. What is the safety significance of the warning buzzer coming on at 50 psi rather than before 60 psi?
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The warning is the driver's last alert before spring-brake application. If it comes on at 50 psi instead of before 60 psi, the usable warning band shrinks to roughly 5–30 psi before the spring brakes apply in the 20–45 psi range. At 55 mph a truck covers about 80 feet per second, so a 10 psi-late warning can mean several hundred feet less distance to reach a safe stop. It also means the warning switch or its adjustment is faulty, so you cannot trust it to warn you at the correct point in the future. For a pre-trip, that is a defect to fix and re-test, not a nuisance to ignore.