Leak Test Failure at the Fargo Terminal
At a Fargo terminal on a January morning, you are pre-tripping a 2019 tractor with a 2016 53-foot dry van. The dash gauge shows 120 psi with the engine off and the parking brakes released. You chock the wheels, release the spring brakes, and time the gauge for one minute: it falls to 116 psi. You then make and hold a firm service brake application; in one minute the gauge drops from 116 to 109 psi. The trailer has red and blue gladhands connected, and you hear nothing obvious. The shop is 12 miles away and your load has a 30-minute delivery window. You have a small adjustable wrench and wheel chocks. The tractor protection valve is in the normal position. The outside temperature is 10°F, and the yard is flat.
1. Do the two leak rates meet the CDL air-brake leak standard? What does the difference between the released and applied rates suggest, and what is your decision?
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No - both readings fail. For a fully charged system, engine off, and parking brakes released, the CDL standard is a pressure loss of no more than 2 psi in one minute. You lost 4 psi. With a firm service brake application held, the limit is 3 psi in one minute. You lost 7 psi. The larger applied-brake loss points to a service-side leak - a brake chamber, diaphragm, pushrod seal, line, or foot valve - in addition to any supply leak. Do not dispatch and do not rely on the compressor to keep up: a compressor can mask a leak at operating rpm, but when the engine is off or demand is high, pressure can fall into the warning and spring-brake range. The 30-minute delivery window does not change the out-of-service condition.
2. How would you isolate the leak between tractor and trailer, and what would you do if the tractor alone held 1 psi released and 2 psi applied?
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Isolate before condemning the whole rig. With the system charged and engine off, close the tractor protection valve or use the trailer shut-off valves so the trailer is isolated, then repeat the one-minute released and applied checks on the tractor alone. If the tractor now holds within 2 psi released and 3 psi applied, the leak is on the trailer side; inspect trailer service lines, gladhands, relay valve, brake chambers, and spring-brake chambers. If the tractor still fails, the leak is on the tractor. In your numbers, a tractor result of 1 psi released and 2 psi applied would pass, so the trailer is the likely source. Even if the tractor passes, the combination is not legal or safe to dispatch with the trailer leaking. Repair the failed side, then repeat the full test before moving.