Leak Test Failure Before Dispatch
You are assigned a 26,000-lb GVWR straight truck with a dual-circuit air system and no trailer. Before departure, you build pressure to governor cut-out at 125 psi and shut the engine off. With the parking brake released, the gauge drops from 120 to 114 psi in 60 seconds. With the service brake fully applied and held, it drops from 120 to 110 psi in 30 seconds. The low-pressure warning buzzer sounds at 60 psi, and the spring brakes apply at about 35 psi. At fast idle, the compressor builds 85 to 100 psi in 60 seconds. The load is time-sensitive, and the shop is 40 miles away on a two-lane road. The truck has recently had air dryer and compressor discharge line work. You must decide whether the leakage is acceptable and what to do before moving.
1. Using the leakage limits for a single vehicle, do these readings pass, and what should you do next?
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No - both tests fail for a single vehicle. With the engine off and the system fully charged, a single vehicle should lose no more than 2 psi in one minute with the parking brake released. Your 6-psi drop in 60 seconds is three times the limit. With the service brake applied, the allowable loss is 3 psi in one minute; 10 psi in 30 seconds equals 20 psi/min, far over. The compressor build is also slow: 85 to 100 psi should take about 45 seconds at fast idle, not 60. Do not dispatch. Chock the wheels, build pressure, shut down, and perform a soap-bubble check at the compressor discharge line, governor, unloader, air dryer, reservoir check valves, fittings, and service-brake chambers. Repair and repeat both tests before release. A slow build plus leakage suggests a supply-side problem, but the applied-brake loss points to a service-side leak as well. Raising the governor cut-out would mask, not fix, the defect.
2. Identify the likely circuits involved and a diagnostic sequence. Why is increasing the governor cut-out not an acceptable fix?
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Start with the most recently disturbed components: air dryer fittings, compressor discharge line, and governor/unloader. Released-brake leak is more likely on the supply side - compressor discharge, governor, unloader, air dryer, check valves, or wet-tank fittings. Applied-brake leak is more likely on the service side - treadle valve, delivery lines, relay valve, brake chambers, or diaphragms. Use a soap solution; look for bubbles and listen at each chamber. Drain tanks and check for compressor oil or water carryover. Do not simply increase governor cut-out: higher pressure increases stress, shortens component life, and leaves a leak that can worsen. Do not drive 40 miles on a time-sensitive load; a leak that fails the static test can drop pressure below spring-brake application on a grade or during repeated stops.