Combination Air Leak Test Failure
After coupling to a loaded trailer at a Bend truck stop, you run the air brake check. With the engine off and the parking brake released, pressure falls from 125 to 119 psi in 60 seconds. You then fully apply the service brake; pressure falls from 119 to 110 psi in the next 60 seconds. The tractor governor cut-in is 100 psi and cut-out is 125 psi. The trailer has spring brakes and ABS. You hear a faint leak near the trailer gladhand, but the trailer brakes hold when you pull the red trailer air supply knob. The low-pressure warning has not been tested today. Your dispatcher wants you to leave now to make the Eugene delivery. You need to decide whether the vehicle passes the combination air-brake leak test, what the likely problem is, and what must happen before dispatch.
1. Does the vehicle pass the combination air-brake leak test? Explain using the measured numbers.
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No, it fails. For a combination vehicle, with the engine off and parking brake released, pressure loss should not exceed 4 psi in one minute; yours is 6 psi. With the service brake fully applied, loss should not exceed 6 psi in one minute; yours is 9 psi. Both exceed the combination limits. The faint leak at the trailer gladhand is a likely source, but do not assume it is the only one. The unit is not legal to dispatch until the leak is repaired and the test is repeated. The dispatcher's schedule does not change the standard.
2. What diagnosis and repair steps would you take?
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Check the trailer gladhand seals first: look for a cut, missing, or swollen rubber seal, and check that the couplers are clean and fully engaged. Then spray soapy water on the gladhands, trailer emergency and service lines, tractor protection valve, relay valve, and brake chambers. If replacing or reseating the gladhand seal fixes the leak, repeat both leak tests. If the leak is at a chamber or valve, repair or replace it. Do not use tape or a substitute seal. After repair, build to governor cut-out, shut off the engine, and repeat the released and applied tests.
3. What other air-brake checks must be completed before moving?
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Also test the low-pressure warning: with the engine off and pressure above 60 psi, pump the service brake until the warning buzzer or light comes on; it must operate before pressure drops below 60 psi. Continue to check that the trailer air supply red knob pops out and the tractor protection valve closes, and that the spring brakes apply at low pressure, typically around 45 psi. Check compressor build time from 85 to 100 psi and governor cut-in and cut-out. Confirm the trailer brakes hold with the red knob pulled. Only after all checks pass should you move.