Combination Vehicle Leak Rate Test
At a Georgia weigh station, a driver performs an air leakage test on a tractor-trailer. The system is charged to 120 psi, the engine is off, the parking brake is released, and the wheels are chocked. In one minute the gauge falls from 120 to 114 psi. The driver then fully applies the service brakes and holds the pedal; in the next minute the gauge falls from 114 to 105 psi. The gauges are mechanical and recently calibrated. The outside temperature is 40°F, and the driver had just come off the highway. The driver has a 250-mile trip ahead. The company standard for combination vehicles is 4 psi per minute for both tests. The driver must decide whether the leak rate is acceptable and what to report.
1. Calculate the leakage rate for each test and determine pass/fail against the 4 psi/min standard. Show the arithmetic.
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Released-brake test: 120 psi minus 114 psi equals 6 psi lost in 1 minute, so 6 psi/min. Against the combination-vehicle limit of 4 psi/min, that fails. Applied-brake test: 114 psi minus 105 psi equals 9 psi lost in 1 minute, so 9 psi/min. That also fails. The two tests must be judged separately; the fact that the total loss over two minutes is 15 psi is not the test. The released test is the lower number and still exceeds the limit by 2 psi/min; the applied test exceeds it by 5 psi/min. A passing system would show no more than 4 psi loss in each one-minute test. This system should not depart.
2. What does the difference between the released and applied readings suggest about where air may be escaping?
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The released-brake leak points to the supply side of the system because the service brakes are not applied. That includes the compressor discharge line, governor, air dryer, reservoirs, fittings, and any valve that is pressurized with the brakes released. The applied-brake leak adds the service-brake circuit: brake chambers, diaphragms, hoses, relay valves, quick-release valves, and the treadle valve. Because the applied leak is larger (9 vs 6 psi/min), the service side is a strong suspect, but the released leak must also be found. A soap-bubble test at the chambers, hoses, and valves is the practical way to locate them. The driver should not assume it is just the compressor from the released test alone.
3. What should the driver report and do before the 250-mile trip, and why is 'only a little over' unsafe?
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The driver should report the exact readings and conditions: 120 to 114 psi in one minute with brakes released, then 114 to 105 psi in one minute with brakes applied, engine off, wheels chocked, at 40°F. The vehicle should be held for repair and retested, not dispatched. 'Only a little over' is unsafe because the leak is present in both tests and will not improve with use. Over 250 miles the driver will apply and release the brakes many times, and each application uses air. A leak that is 6 to 9 psi/min at rest can grow with vibration, temperature change, or a failing diaphragm. If pressure falls faster than the compressor can recover it, the low-pressure warning will activate and the spring brakes can apply. The margin between 4 psi/min and the observed rates is the safety margin, not a technicality.