Combination Leak Test Fails at Denver Yard
At your Denver terminal at 5,280 ft on a 35°F morning, you perform a pre-trip leak check on a 3-axle tractor and 2-axle trailer. You build pressure to 120 psi, shut off the engine, and wait for the gauge to stabilize. With the parking brakes released and the service brakes released, pressure falls from 120 to 114 psi in 1 minute. You then fully apply the service brakes and hold them; pressure falls from 120 to 112 psi in 1 minute. The gauge is mechanical and recently calibrated. You hear no obvious leak, and the tractor protection valve has not popped. The trailer has spring brakes and the glad hands are coupled. You are scheduled to leave in 20 minutes with a 44,000 lb load. The truck passed an annual inspection 10 months ago.
1. Apply the CDL leak-rate limits for a combination vehicle and determine pass or fail for both tests.
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For a combination vehicle, the allowable pressure loss with the engine off and brakes released is no more than 3 psi per minute. With the service brakes fully applied, the allowable loss is no more than 4 psi per minute. Your released-brake test lost 6 psi in 1 minute, which is double the limit. Your applied-brake test lost 8 psi in 1 minute, also double the limit. Both tests fail. The fact that the annual inspection was 10 months ago does not matter; a leak can develop at any time. The gauge calibration and the lack of an audible leak do not excuse the measured loss. The vehicle should not be dispatched until the leak is found and repaired.
2. Describe a systematic method to isolate the leak and list the components you would check.
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Chock the wheels first. Build full pressure, shut off the engine, and leave the parking brake released for the first check. Use soapy water at likely leak points and listen carefully. Start at the glad hands and trailer supply and emergency lines, then check the tractor protection valve, relay valve, spring brake chambers, service chambers, air dryer, drain valves, fittings, and hoses. A leaking chamber diaphragm may show air at the pushrod opening or breathing holes. If the leak is hard to locate, disconnect the trailer and retest the tractor; if the tractor holds pressure, the trailer is the likely source. Recheck both the released-brake and applied-brake tests after any repair. Do not rely on hearing alone, because a 6 to 8 psi per minute leak can be quiet in a noisy yard.
3. What must happen before dispatch, and may you drive 3 miles to a shop for repair?
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The vehicle must be repaired and must pass both leak tests within the combination limits before it is dispatched. A 3-mile trip to a shop is not automatically acceptable when the leak exceeds the standard by this much. The safe and compliant choice is to have the repair performed on site or have the vehicle towed. Driving with a known 6 to 8 psi per minute leak risks losing pressure on the road, especially if the leak worsens or the compressor cannot keep up. If a mechanic authorizes a short movement, it should be treated as a maintenance move with controlled conditions, not as a normal dispatch. Document the failed readings and the repair.