Low-Air Warning and Leak Test
During a pre-trip inspection on a Class A combination vehicle, you start the engine and let air pressure build. The governor cuts out at 120 psi and cuts in at 100 psi. With the engine off and the parking brake released, you let the pressure stabilize and watch the gauge for one minute; pressure drops from 118 to 115 psi. You then fully apply the service brakes and hold the treadle valve; pressure drops from 115 to 107 psi in one minute. The low-air warning buzzer sounds at 60 psi, and the spring brakes set at 45 psi. You also notice that during the applied test the dash gauge for the front circuit drops faster than the rear-circuit gauge. The vehicle is loaded to 78,000 lb, and you are scheduled for a 250-mile interstate run. Your company wants you to depart immediately.
1. Interpret the leak-test results. Is the vehicle safe to dispatch?
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No. The unapplied leak rate is marginal but the applied leak rate fails. For a combination vehicle, with the engine off and the brakes released, air loss should not exceed 3 psi per minute; with the service brakes fully applied, it should not exceed 4 psi per minute. Your released test lost 3 psi in one minute, which is at the limit. Your applied test lost 8 psi in one minute, double the allowable rate.
The faster drop on the front-circuit gauge during the applied test suggests a leak in that circuit, such as a chamber, hose, valve, or fitting. A vehicle with an excessive applied leak can lose pressure during a long downgrade or repeated brake use, reducing braking performance and eventually triggering the low-air warning. The correct decision is do not dispatch until the leak is found and repaired and the test passes.
2. What components should be checked, and what records or actions should follow?
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The leak must be localized, not guessed at. Check:
- Gladhands and trailer lines for seals, cracks, and improper coupling.
- Service and emergency lines along the tractor and trailer for chafing or loose fittings.
- Brake chambers and diaphragms for ruptures or leaks.
- Relay valves, quick-release valves, and the tractor protection valve.
- Compressor and governor operation, though the cut-in/cut-out pressures here are normal.
After repair, repeat the static leak test with the engine off, brakes released, and brakes applied, and confirm the low-air warning and spring-brake application. Record the defect and the repair on the inspection report. If the vehicle is placed out of service, notify the carrier and do not move it until the brake system is safe.
3. Are the low-air warning and spring-brake set points acceptable?
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Yes, those two readings are acceptable. The low-air warning must activate before pressure drops below 60 psi; yours sounds at 60 psi, which meets the requirement. The spring brakes must apply when pressure falls to the 20–45 psi range; yours set at 45 psi, at the top of the acceptable range.
However, acceptable warning and spring-brake points do not excuse the failed applied leak test. The warning system is a backup; it is not a substitute for a sealed brake system. The vehicle still cannot be dispatched until the excessive leak is corrected.