Pre-Trip Air Build-Up and Leak Test
You are doing a pre-trip inspection on a 2016 Freightliner tractor with a 53-foot trailer. At fast idle, system pressure rises from 85 to 100 psi in 70 seconds. The governor cuts out at 140 psi and cuts back in at 105 psi. With the engine off and parking brakes released, pressure drops from 120 to 114 psi in one minute. With the service brakes fully applied and held, pressure drops from 118 to 113 psi in one minute. The compressor cycles frequently, and you hear a faint hiss near the air dryer, but no obvious line damage. The vehicle is loaded and scheduled for a 300-mile run. The dispatcher says the truck 'just passed last month.' You are the only driver available.
1. Which readings are outside the normal range, and why does each matter?
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The 70-second build-up from 85 to 100 psi is too slow; the standard is within 45 seconds at fast idle. The governor cut-out at 140 psi is too high; normal cut-out is about 120-135 psi. The released-brake leak is 6 psi per minute (120 to 114). For a combination vehicle the limit is 4 psi per minute with brakes released, so this fails. The applied-brake leak is 5 psi per minute (118 to 113). For a combination the limit is 6 psi per minute with brakes applied, so that reading alone is within limits. The frequent compressor cycling and hiss near the dryer support a real leak, not gauge error.
2. Can you legally and safely dispatch the vehicle? What would you do?
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Do not dispatch. Three defects are out of specification: slow build-up, high governor cut-out, and excessive released-air leak. The applied-leak reading being acceptable does not cancel the released leak; a leak can behave differently as valves shift. The faint hiss near the dryer is a clue to inspect the dryer, purge valve, and lines. The 'passed last month' sticker is irrelevant to today's condition. Document the readings, place the vehicle out of service, and have a qualified mechanic repair and re-test. A 300-mile loaded run is exactly where a worsening leak could leave you with fading pressure and automatic spring-brake application.