Leak Test Fails at the Terminal Gate
You are doing a pre-trip inspection on a 3-axle tractor with a 2-axle semi-trailer (combination). Air pressure is fully charged to 120 psi; engine off; parking brakes released; service brakes released. Over 60 seconds, the dash gauge falls from 120 to 115 psi. You then apply and hold the service brake; pressure falls from 115 to 108 in 60 seconds. The trailer has an air dryer purge that released a burst just before the test. Dispatch says the truck passed a shop check yesterday. You have a 300-mile run. You must decide whether the combination passes the leak test, what the allowable losses are, and what to do next. The outside temperature is 35°F, and you are already 15 minutes behind schedule.
1. Did this combination pass the static leak test? Give the limits and your reasoning.
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No. For a combination vehicle, with the engine off and the parking brakes released, the allowable pressure loss is about 3 psi in one minute; with the service brake applied and held, it is about 4 psi in one minute. This truck lost 5 psi in the released test and 7 psi in the applied test. Both exceed the limits. The air dryer purge just before the test is normal and should not be counted as a leak, but the gauge did not stabilize after the purge; the steady drop over the full minute is the problem. The shop check from yesterday does not override today's measurement. A failed leak test means the vehicle is not safe for the 300-mile run.
2. What should you do next, and what should you not do?
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Do not dispatch. Secure the vehicle, set the parking brake, and report the failure. Recheck the test correctly: full pressure, engine off, wait for the air dryer to finish purging and the gauge to stabilize, then observe for one minute with brakes released, then one minute with the service brake applied. Look for obvious causes such as a gladhand not seated, a cracked line, a leaking chamber, or a valve leak. Do not fix it by raising the governor cut-out or by ignoring a small leak; the limits exist because a leak that is small at rest can grow with heat, vibration, and brake use. If the leak cannot be found and corrected, the vehicle should be repaired by a qualified technician and retested.
3. Why is the applied leak test important even though the released test already failed?
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The applied test checks different passages and seals. The released test can miss a leak that opens only when the treadle valve or relay valve is pressurized. Here the applied loss is larger, which points to a leak in the service side, such as a chamber, hose, or valve that leaks only when air is delivered. Recording both results helps the technician locate the fault, and it documents that the vehicle failed both parts of the inspection. For the driver, the practical point is the same: a combination that loses that much air in one minute will not maintain safe pressure during a run with repeated brake applications.