Air Leak Test Fails in Cold Weather
At a pre-trip inspection on a 2019 tractor-trailer with dual air circuits, you build pressure to 120 psi and shut the engine off. With the parking brake released and the service brake fully applied, the gauge drops from 120 to 114 psi in the first minute and to 108 psi in the second. You then release the service brake and watch: pressure falls from 108 to 104 in one minute. The low-pressure warning buzzer comes on at 58 psi, and the spring brakes set at 42 psi. The compressor cuts in at 100 psi and out at 120 psi. The tractor has a 12-volt electrical system and an air dryer. It is 15°F outside and there is a light coating of ice on the trailer gladhands. You are scheduled to leave in 20 minutes with a 300-mile run.
1. Apply the standard leak tests to these numbers. Do they pass, and what do they point to?
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For a combination vehicle, leakage with the service brake applied must not exceed 4 psi per minute. Here it is 6 psi per minute: 120 to 114 in the first minute, then another 6 in the second. With the brakes released, leakage must not exceed 3 psi per minute; here it is 4 psi. Both tests fail.
The warning buzzer and spring brakes are in normal ranges-warning before 60 psi and spring brakes between 20 and 45 psi-so those are not the failed item. A leak present both applied and released suggests a supply-side leak: gladhand seals, a hose, a fitting, or a brake chamber diaphragm, possibly made worse by ice. Do not assume the compressor will cover it.
2. Can you make the 300-mile run? Explain what happens to the compressor and brake system if you do.
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No. At 5 to 6 psi per minute, the compressor will cycle from cut-in at 100 to cut-out at 120 every few minutes. It may not keep up, especially at low engine speed, and it will run hot and load the air dryer with moisture. Pressure could fall to the warning and spring-brake thresholds during a long stop or heavy brake use.
Cold weather plus ice on the gladhands raises the risk of a frozen leak or a brake that drags or releases slowly. The vehicle is not roadworthy until the leak is found and repaired. A 20-minute schedule is not a reason to go; the safe choice is to notify dispatch and get the repair done.
3. What cold-weather checks and repairs would you make before the run?
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Drain the tanks, check the air dryer and purge valve, check the alcohol evaporator if equipped, and inspect the gladhand seals for cracks or ice. Replace the trailer gladhand seals. Soap-test hoses, fittings, and chambers to locate the leak.
Do not use an open flame to thaw a frozen line. If a chamber diaphragm is leaking, replace it; if a hose is damaged, replace it. After repair, repeat both leak tests-applied and released-and confirm that the trailer spring brakes apply and release fully. Also check that the low-pressure warning still activates before 60 psi.