In-Cab Air Brake Tests After Preventive Maintenance
After a PM service on a 2015 Freightliner Cascadia day cab, a single-unit truck with no trailer, you perform the in-cab air brake tests. At fast idle, pressure builds from 85 to 100 psi in 3 minutes 50 seconds. The governor cuts out at 123 psi and cuts in at 101 psi. With the engine off, parking brake released, and no brakes applied, pressure drops from 120 to 118 psi in one minute. With the engine off and the brake pedal fully applied and held, pressure drops from 115 to 108 psi in one minute. You pump the pressure down; the low-pressure warning buzzer comes on at 57 psi; the spring brakes apply at 34 psi. The truck is needed for a 200-mile run in 30 minutes.
1. Evaluate each test result against the standard in-cab air brake checks.
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The build time fails. At fast idle, the compressor should build 85 to 100 psi within 3 minutes; this truck takes 3 minutes 50 seconds, so it builds too slowly. The governor cut-out at 123 psi and cut-in at 101 psi are in the normal range. The unapplied leakage test passes for a single vehicle: a 2 psi loss in one minute is below the 3 psi limit. The applied leakage test fails: a 7 psi loss in one minute is above the 3 psi limit for a single vehicle. The low-pressure warning at 57 psi passes because it must come on before 60 psi. The spring brakes applying at 34 psi pass because they should apply in the 20-45 psi range.
2. What does the applied-leak result suggest, and can the truck make the 200-mile run?
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The applied leak is the most serious result. When the pedal is held down, air is escaping through the service brake circuit, such as a brake chamber diaphragm, a valve, a line, or a fitting. A 7 psi per minute loss is more than double the 3 psi limit, and it may get worse as the brakes heat and seals expand or wear. The slow build time also shows the compressor or charging system is not performing correctly. Together, these results mean the truck is not roadworthy. It should not make the 200-mile run or even a short trip until the leak and slow build are repaired.
3. What should you do before this truck is released, and what should you avoid doing?
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Red-tag the truck and write the failures on the DVIR. Have a technician find the applied leak with soapy water at the chambers, relay valves, treadle valve, lines, and fittings, and diagnose the slow build time. Repair the defective parts, then repeat the complete in-cab test: build time, governor cut-in and cut-out, unapplied leak, applied leak, low-pressure warning, and spring-brake application. Avoid turning up the governor or disabling the warning to make the tests look better. Those changes do not fix the leak and can hide a condition that causes brake loss on the road.