Pre-Trip Inspection: Air Leak at the Rear Axle
It is 06:10 on a cold February morning, and you are assigned a 54-passenger coach for a 07:00 charter. The bus sat outside overnight at -8°C. During your pre-trip inspection you build air pressure to 105 psi, shut off the engine, and release the parking brake. With the service brakes released, pressure falls from 105 to 92 psi in 2 minutes. With the service brake pedal fully applied, it falls from 100 to 78 psi in 1 minute. You hear a steady hiss near the rear axle. The low-pressure warning buzzer has not sounded. The bus starts and drives normally in the yard. Dispatch tells you the charter is a 30-minute drive to the pickup point and the group cannot be late. The previous driver’s daily inspection report shows 'air system - OK' from yesterday.
1. Interpret the leak-test readings. Are they acceptable, and does the fact that the buzzer has not sounded change your decision?
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The readings are not acceptable. As a general benchmark, with the engine off and service brakes released, pressure loss should not exceed about 2 psi per minute; with the brakes applied, it should not exceed about 3 psi per minute. Here pressure fell 13 psi in 2 minutes with brakes released, which is about 6.5 psi per minute, and 22 psi in 1 minute with brakes applied. Both are far beyond allowable, and the steady hiss confirms an active leak.
The buzzer not sounding only means pressure has not yet fallen below the warning threshold, typically around 60 psi. It does not mean the system is safe. A leak this size can worsen quickly, reduce braking performance, and cause the spring brakes to apply unexpectedly. The bus must not be dispatched.
2. What must you do before the bus can be dispatched, including documentation and communication with dispatch?
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Record the defect on the daily inspection report and report it to dispatch or the operator immediately. The bus must be placed out of service and cannot be driven on the charter until the leak is repaired by a qualified person and the air system is re-inspected and leak-tested. The previous day’s 'OK' report is irrelevant because the defect is present now.
Do not take the bus to the pickup point to 'have it looked at later.' Arrange an alternate bus, delay the charter, or have the repair done before any passenger is loaded. Document the defect, the repair, and the re-test. If the bus is moved at all, it should be only as necessary for repair, not in passenger service.
3. If dispatch pressures you to drive the 30 minutes and 'have it looked at after,' how do you respond, and what is the risk?
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Refuse to drive the bus in passenger service. Explain that the leak rates are well beyond allowable and that air loss can lead to loss of service brakes and automatic spring-brake application, possibly at highway speed with 54 passengers aboard. That is a direct safety risk, not a minor maintenance item.
Offer alternatives: use another bus, delay the pickup, or call a mechanic. If dispatch insists, escalate to a supervisor and note the refusal and the defect in writing. Ontario law and the driver’s duty require a safe vehicle; schedule pressure does not override an unsafe air system.