Slow Build, Low Cut-Out, and a Milky Wet Tank
At a cold Maryland morning start, a 2018 straight truck with air brakes has been sitting overnight. The temperature is 28°F. The engine starts, but pressure builds from 0 to 70 psi in 4 minutes. The governor cuts out at 102 psi and cuts in at 68 psi. The air dryer purges every 20 seconds. When you drain the wet tank, a milky emulsion comes out, followed by clear air. With the engine off and brakes released, pressure falls from 100 to 82 psi in 5 minutes. The truck is due for a 120-mile delivery. The shop is closed until noon; it is 07:00. The driver before you wrote “slow build, normal” on the log.
1. Which readings are abnormal, and what systems do they point to?
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Several readings are abnormal. Building only to 70 psi in 4 minutes is slow, and the governor cut-out at 102 psi is low; typical cut-out is around 120–125 psi. A cut-in at 68 psi is also low; typical cut-in is near 100 psi. The air dryer purging every 20 seconds suggests frequent compressor cycling or a leak/air-dryer problem. The milky wet-tank drain indicates moisture, possibly with oil, in the air supply. With the engine off and brakes released, pressure falling from 100 to 82 psi in 5 minutes-18 psi-is an excessive leak. Together these point to the compressor/governor, air dryer, and/or air circuit, not just a gauge problem.
2. Is it safe to make the delivery? Explain the risk at 28°F.
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No. The truck has a slow build, low cut-out and cut-in, excessive leakage, and moisture in the wet tank. At 28°F, that moisture can freeze in lines, valves, or the air dryer and block or slow air flow. A low cut-out already gives less reserve pressure; if the compressor cannot keep up or a leak worsens, pressure can fall toward spring-brake application while the truck is moving. The previous driver's 'slow build, normal' note does not make it acceptable. The delivery should be assigned to another truck or delayed until the air system is repaired and tested.
3. What immediate and maintenance actions should follow?
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Do not dispatch the truck. Drain all reservoirs-wet, primary, and secondary-and note what comes out. Have the shop service the air dryer, check the compressor and governor, and perform both applied and unapplied leak tests. The low cut-out, low cut-in, and slow build must be corrected, not just adjusted blindly; the moisture source and any leak need to be found. After repair, retest build time, cut-in/cut-out, warning activation before 60 psi, spring-brake application and release, and leak rate. Document the original readings and the repairs. If the shop cannot complete the work before the delivery window, use a spare unit.