Post-Repair Leak Test and Push-Rod Travel on a Combination
A combination vehicle returns from a brake job. The shop replaced the right rear brake chamber and adjusted the slack adjusters. Before signing the inspection report, you build system pressure to 120 psi, shut the engine off, chock the wheels, release the parking brake, and hold a full service brake application for one minute. The gauge falls from 120 to 114 psi. You also measure push-rod travel on the left front chamber: it is 2 inches; the chamber stamp says the adjustment limit is 1¾ inches. The low-pressure warning comes on at 59 psi and the spring brakes apply at 40 psi. The governor cuts in at 100 psi and out at 125 psi. The vehicle is a 3-axle tractor with a two-axle trailer. The dispatcher needs it for a 300-mile run in 30 minutes.
1. Is the 6 psi drop in one minute acceptable for this combination? Explain the test and the limit.
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No. For a combination vehicle, a fully applied service-brake leak test typically should not show more than 4 psi loss in one minute; the usual limit for a single vehicle is 3 psi. Your gauge fell 6 psi, so the test fails. This is not normal gauge fluctuation. Because the leak appears with the service brake applied, it points to a leak in the service-brake circuit-possibly the newly installed chamber, a fitting, a line, or a valve. An unapplied leak test alone would not rule this out. The vehicle should not be signed off or dispatched until the leak is found and repaired, then the applied leak test is repeated and passes. The passing governor and warning readings do not offset a failed leak test.
2. What do you conclude from the 2-inch push-rod travel against the 1¾-inch limit?
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The left front brake is out of adjustment. A 2-inch push rod exceeds the chamber's marked 1¾-inch limit, so the vehicle is not roadworthy as presented. If the slack adjuster is manual and you are qualified, adjustment may be part of the repair-but adjustment alone is not enough if the brake was just serviced and has already moved out of adjustment. That suggests an installation problem, a mismatched chamber, or a foundation-brake issue. If the slack adjuster is automatic, do not manually adjust it to hide the symptom; find out why it is out of adjustment. Either way, the vehicle should be held out of service until the cause is corrected and push-rod travel is within the marked limit.
3. The low-pressure warning, spring brakes, and governor readings pass. What is your next step?
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Do not release the vehicle just because some checks pass. The failed applied leak test and the excessive push-rod travel are out-of-service defects. Tell the dispatcher the unit cannot make the 300-mile run. Have the shop repair the air leak and correct the left front brake adjustment, then repeat the full inspection: build to cut-out, check governor cut-in/cut-out, perform the applied and unapplied leak tests, verify low-pressure warning before 60 psi, verify spring-brake application and release, and re-measure push-rod travel. Document the original readings, the repairs, and the passing retest. If the shop cannot complete the work in time, assign a different tractor-trailer.