Trailer Service-Line Leak on a Pre-Trip
At a Rochester, Minnesota truck stop, you pre-trip a 2018 Kenworth T680 pulling a 2020 Great Dane trailer. Engine off and parking brakes released, the gauge holds at 120 psi for three minutes. You hear a faint hiss near the trailer gladhands. When you apply the foot valve, the hiss becomes a steady roar and pressure falls from 120 to 90 psi in 45 seconds. Soap bubbles appear at the trailer service-line gladhand, not the supply line. The trailer spring brakes are not set. The low-air warning has not sounded, and the tractor protection valve has not popped. You are 18 miles from a repair shop and 60 miles from the delivery. Your co-driver says the leak is minor because the trailer brakes still hold when parked. The appointment is in 90 minutes.
1. Why did the static test pass while the applied test failed, and what does that tell you?
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The service line is not pressurized when the brakes are released, so a leak there does not show up in the static test. When you apply the foot valve, air flows to the trailer brake chambers through the service line, and the leak opens. Bubbles at the service gladhand confirm the leak is on the trailer service side, not the supply side. The static test passing is therefore not clearance to drive; the applied test is the one that exposes this defect.
2. What are the consequences of driving with this leak?
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A service-line leak reduces the air delivered to the trailer brakes, so trailer braking is weak or delayed and stopping distance increases. If the leak is large, the compressor may not keep up and pressure can fall toward the low-air warning and spring-brake application. The tractor brakes may still feel normal, which makes the defect easy to underestimate. The co-driver's parked test does not prove the trailer will brake in a stop.
3. What should you do before the truck moves?
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Do not drive to the shop or the delivery. First try reseating both gladhands and inspect the coupler seal and gladhand for cracks or missing rubber; replace the seal if you have one. If the leak persists, call mobile repair. After repair, build air pressure, shut the engine off, apply the brakes, and confirm leakage is within limits, then do a low-speed trailer brake test. Document the defect and the repair.