The Leak Test That Doesn't Add Up
You are pre-tripping a three-axle tractor pulling a two-axle van trailer - a combination vehicle. The air system is fully charged to 120 psi. With the engine off and the parking brakes released, you let the system stabilize, then watch the gauge for one minute. It drops from 120 to 118 psi. Next, you hold the service brake pedal down and watch for another minute; the gauge drops from 118 to 114 psi. The governor cuts out at 125 psi and cuts in at 95 psi. The low-pressure warning buzzer comes on at 58 psi, and the spring brakes apply at 42 psi. You have a 180-mile run and a 06:00 appointment. The dispatcher says the truck “passed the leak test last week” and tells you to get rolling.
1. Interpret the two leak-test results against the combination-vehicle standard.
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For a combination vehicle, the engine-off, brakes-released leak limit is 3 psi per minute; a 2 psi drop from 120 to 118 psi is within that limit. The applied-brake leak limit is also 3 psi per minute, but the gauge drops 4 psi from 118 to 114 psi, which exceeds the limit. That is a failed leak test. The first-minute drop can include some cooling and stabilization, which is why the test is read after the system settles; the applied leak is the hard failure here. The governor cut-out and cut-in pressures are in a normal range, but that does not excuse the applied leak. The low-pressure warning at 58 psi is close to the 60 psi target and should be checked, but the decisive defect is the 4 psi applied leak.
2. What is the likely consequence of ignoring the failed applied-brake test, and what should the driver do before moving?
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An applied-brake leak means air is escaping when the service brakes are held, so the system may not maintain enough pressure for repeated or sustained braking. On a long downgrade or in heavy traffic, the leak can worsen, cause brake fade, or trigger the low-pressure warning and spring brakes at the worst time. The driver should not move the vehicle. Park it safely, set the parking brakes, tag or lock it out according to company policy, and report the failed test to maintenance. The repair must be made and the leak test repeated before dispatch. A dispatcher’s statement that the truck passed last week is not evidence about the current condition; air leaks can develop suddenly. The 180-mile run and 06:00 appointment do not justify driving an unsafe brake system.
3. If this were a single-unit truck instead of a combination vehicle, would the released-brake result pass?
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The single-unit released-brake limit is stricter: no more than 2 psi per minute. A drop from 120 to 118 psi is exactly 2 psi, so it would be at the maximum allowed and would pass only if it did not exceed 2 psi. The applied-brake result would still fail because the limit remains 3 psi per minute and the observed drop is 4 psi. The comparison matters because the driver must apply the correct standard to the vehicle being tested; using the combination limit on a single unit could wrongly excuse a leak. In either case, the applied leak is the controlling defect.