Pre-Trip Air Brake Failure Before the Road Test
You are at the Kansas CDL skills-test site in a 72-passenger school bus with air brakes. Before the road test, you perform the pre-trip inspection. With the engine running at 700 rpm, you build pressure from 85 to 100 psi. It takes 68 seconds. You then shut off the engine and perform a static leak test: pressure drops 2 psi in one minute with the brakes released. The low-air warning buzzer comes on at 55 psi, and the spring brakes set at 32 psi. The examiner is waiting, and another candidate is scheduled in 20 minutes. The bus otherwise starts, steers, and brakes normally on the lot. You have no spare bus at the site. You must decide whether to take the bus on the road test.
1. Evaluate each result against the CDL standard and decide whether the bus is safe for the road test.
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The build-up test fails: with the engine at 600 to 900 rpm, pressure should rise from 85 to 100 psi within 45 seconds. This bus takes 68 seconds. The static leak test passes: a loss of 2 psi in one minute with the brakes released is within the 3 psi limit for a single vehicle. The low-air warning test fails: the warning must come on before pressure falls below 60 psi, and this one comes on at 55 psi. The spring-brake test passes: the parking brakes should set between 20 and 45 psi, and 32 psi is in range. Because the build-up and warning tests fail, the bus is not safe for the road test. It should be placed out of service.
2. What do you do about the examiner and the 20-minute schedule?
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I would tell the examiner the specific failed results and ask for the bus to be repaired or replaced. I would document the defect and notify the shop or carrier. I would not take the bus on the road just to keep the schedule; the air-brake tests are pass/fail for a reason, and a slow build or late warning could become an emergency on the route. If no spare bus is available, the road test cannot proceed with this vehicle. The examiner can reschedule or use another vehicle; the schedule does not override the defect.
3. If the build had taken 40 seconds and the warning had come on at 62 psi, with the same leak and spring-brake results, would you proceed?
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Those two results would pass: 40 seconds is within 45, and 62 psi is before the 60 psi threshold. The 2 psi leak and 32 psi spring-brake setting would also pass. I would still complete the rest of the pre-trip inspection and the brake tests, because passing four items does not clear the whole bus. If everything else is satisfactory, I could proceed to the road test. The judgment changes because the specific defects that made the bus out of service are gone, not because the schedule is tighter.