Pre-Trip Leak Test and Pushrod Measurement in Gallup
You are doing a pre-trip inspection on a three-axle tractor with a two-axle semitrailer, GCW 78,000 lb, in the truck stop lot in Gallup before a run to Phoenix. You build system pressure to 100 psi, shut the engine off, release the parking brakes, and wait one minute. The gauge falls from 100 to 97 psi. You then press and hold the service brake pedal for one minute; the gauge falls from 97 to 91 psi. You measure the left rear drive-axle chamber, which is a clamp-type Type 30 with a manual slack adjuster: the pushrod extends 1 3/8 inches from the chamber when the brake is applied. The trailer chamber on the same side measures 1/2 inch. The low-air warning works, and the compressor builds from 85 to 100 psi in 40 seconds at 1,000 rpm. The truck has no ABS light. Your dispatcher wants you to roll.
1. Evaluate the two leakage tests against CDL limits. What is your pass/fail decision and why?
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Static test: the gauge fell 3 psi in one minute. For a combination vehicle the limit is 4 psi, so that part passes, although it is close enough that I would watch it. Applied test: the gauge fell 6 psi in one minute while the service brake was held. That exceeds the 4 psi limit and fails. A leak that appears only with the pedal applied points to a service chamber, diaphragm, hose, or valve in the service brake system. I would not sign the vehicle off. The dispatcher's schedule does not override the limit. The correct decision is to find and repair the leak, then repeat both tests before dispatch.
2. Evaluate the pushrod travel readings. What action is required before the truck can leave?
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The left rear drive-axle pushrod extends 1 3/8 inches. On a standard clamp-type Type 30 chamber the limit is 1 inch, so this chamber is out of adjustment. The trailer at 1/2 inch is within limits. I would first verify the chamber type and size because long-stroke chambers allow more travel, but this setup states a standard Type 30. Because the slack adjuster is manual, it can be adjusted, and I would re-measure after adjusting. If it will not hold the adjustment, the slack adjuster or chamber is worn and must be repaired or replaced. Either way the tractor is out of service until the pushrod travel is within limit. The two failures together - an applied-air leak and an over-travel pushrod - mean the truck should not roll.