Leak Test Numbers Out of Limits on a Combination Vehicle
At the end of the day you perform the air brake leak tests on your combination vehicle, a three-axle tractor with a tandem-axle van trailer. You build the system to 120 psi, shut off the engine, chock the wheels, release the parking brake, and let the pressure stabilize. With the brakes released, the gauge falls from 120 to 114 psi in one minute. You then recharge to 120 psi, fully apply the brake pedal, and hold it; the gauge falls from 120 to 112 psi in one minute. The tractor has no obvious hissing, and the trailer is a 2016 model with spring brakes. Outside temperature is 35°F. Your co-driver says the numbers are close enough because "air brakes always leak a little." The company's maintenance form asks you to state pass/fail and describe the leak location before dispatch.
1. State whether each test passes or fails for a combination vehicle and cite the limits.
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Both tests fail. For a combination vehicle, the unapplied (static) leak test allows no more than 4 psi loss in one minute; you lost 6 psi (120 to 114). The applied test allows no more than 6 psi per minute for a combination; you lost 8 psi (120 to 112). The standard test is done at full pressure, engine off, wheels chocked, and-for the static test-parking brakes released so all circuits are charged. The applied test must start from a fully recharged system, which you did. Your co-driver's "they all leak" view is wrong: these limits exist because air loss compounds, and a leak that is small at 120 psi can become a runaway loss at the worst moment.
2. A leak that appears only when the brakes are applied is most likely where? Give two candidate locations and how you would find it.
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A leak that shows up only when the pedal is applied is in a component pressurized only during a brake application. Prime suspects are a rear brake chamber diaphragm, a chamber hose or fitting, a relay valve, or the foot/treadle valve. If the leak were in a supply line or tank, the static test would also fail. Find it with soapy water: hold a full brake application, then spray each chamber, hose end, fitting, relay valve, and the foot valve, looking for bubbles. You can also listen with the engine off. Do not "test" by crawling under an unblocked vehicle-use wheel chocks and a second person.
3. Why does the cold temperature matter, and what would you do before dispatch?
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At 35°F, any moisture in the system can freeze in lines or valves, and a small leak can become a blocked or stuck valve. Before dispatch: repair the leak, then repeat both tests from a fully charged system and confirm the loss is within limits. Drain the air tanks and check the air dryer for a proper purge; if the dryer is not working, moisture will keep entering and freezing. Verify the alcohol evaporator (if equipped) is serviced. A failed leak test is an out-of-service condition, not a "note it on the form" item.