Released Test Passes but Applied Test Fails
You are the driver of a tractor-trailer with three axles on the tractor and two on the trailer. After building full pressure, you park on level ground, chock the wheels, shut off the engine, and release the parking brakes. The gauge reads 122 psi. After exactly 2 minutes it reads 115 psi. You then press and hold the brake pedal firmly. The gauge reads 120 psi at the start; after 1 minute it reads 111 psi. You hear a faint hiss near the drive axle but cannot see a leak. The trailer is empty, the roads are dry, and your dispatcher wants you to leave immediately for a 90-mile run. The shop foreman says a little leakage is normal on an older unit and suggests topping off the tanks and going.
1. Evaluate both tests against the standard leakage limits. Does the vehicle pass or fail overall?
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Released-brake test: 122 minus 115 equals 7 psi lost over 2 minutes, or 3.5 psi per minute. For a combination vehicle, the released-brake limit is 4 psi per minute, so that part passes. Applied-brake test: 120 minus 111 equals 9 psi lost over 1 minute, or 9 psi per minute. The applied limit is 4 psi per minute, so this fails. The vehicle fails overall because one required test is more than double the allowable leakage rate. A released-brake pass does not cancel an applied-brake failure.
2. Where is the leak likely to be, and why did it appear only when the pedal was applied? What inspection steps would you take?
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The leak is in the service-brake circuit, because the released test was acceptable while the applied test failed. With the parking brakes released, the service chambers are not pressurized; pressing the pedal sends air to them, so a leaking service diaphragm, hose, fitting, or valve will show up only then. The faint hiss near the drive axle points to that axle's chamber diaphragm or a hose/fitting there. Use soapy water at the chamber clamp, diaphragm, hoses, fittings, relay valve, and treadle area, and check for a cracked or loose line. Do not simply tighten everything and assume it is fixed; identify the actual source.
3. The dispatcher wants the load to move now. What is your decision, and what must happen before dispatch?
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Do not dispatch. Topping off the tanks does not repair a 9 psi per minute service leak; at highway speed the compressor may mask it, but the leak can worsen and the system can drop into low-pressure warning and spring-brake application. Repair the leaking component, then repeat both leak tests with the engine off, the vehicle fully charged, and the wheels chocked. The vehicle should not leave until the applied leakage is at or below 4 psi per minute and the released leakage is within the combination limit.