Cold-Morning Air Brake Leak Test on a Transit Bus
At 6:10 a.m. on a 12°F morning in Bangor, you are assigned a 40-foot transit bus with 30 passengers scheduled for a 6:30 departure. During the pre-trip, the air governor cuts out at 120 psi. With the engine off and the service brakes released, the gauge drops from 118 to 116 psi in one minute. With the service brakes fully applied and held, it drops from 120 to 114 psi in one minute. As you continue, the low-air warning buzzer and light activate at 52 psi, and the spring brakes apply at 35 psi. The compressor builds from 85 to 100 psi in about 40 seconds. Your dispatcher radios that the spare bus is two hours away and tells you to 'top it off and roll-the leak is small.'
1. Which pre-trip results pass or fail, and against what standards?
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Two results fail. (1) Applied-brake leak test: with the service brakes fully applied and held, a single-unit vehicle may lose no more than 3 psi in one minute. This bus lost 6 psi (120 to 114), so it fails. (2) Low-air warning: the warning must come on before pressure drops below 60 psi. Here it first sounded at 52 psi, so it fails. The released-brake test lost 2 psi in one minute, which is at-not over-the 2 psi limit for a single vehicle, so that result passes. Spring brakes applying at 35 psi is normal (the range is 20–45 psi), and the compressor build rate is acceptable. The bus is not roadworthy because the applied circuit is leaking at twice the allowable rate and the warning is late.
2. May you take the bus with passengers? What must you do before it moves?
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You may not take the bus with passengers, and topping it off does not fix either defect. The compressor can keep the gauge near 100–120 psi at idle, but a 6 psi/min leak in the applied circuit will drain pressure during a long brake application-a downgrade, heavy traffic, or a panic stop. If pressure falls to 35 psi, the spring brakes will apply on their own, possibly in a live lane. A late low-air warning removes your only advance notice. Tag the bus out of service, record the readings and outside temperature on the DVIR, notify maintenance, and request the spare bus or another carrier. If the spare is two hours out, the passengers wait or the trip is rescheduled; the schedule does not override the brake standard.
3. What should you tell the dispatcher, and what if the dispatcher insists?
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Tell the dispatcher the specific failures: 6 psi applied leak versus a 3 psi limit, and warning at 52 psi versus a 60 psi requirement. Put the refusal and the readings in writing (text or DVIR), and ask for the replacement in writing. If pressured, remember that the driver has final authority on whether the vehicle is safe, and a dispatcher cannot order you to drive an out-of-service bus. Do not sign a DVIR that says the bus is satisfactory. If the company persists, escalate to safety, and do not move the bus. The downside of waiting two hours is inconvenience; the downside of a brake failure with 30 passengers is not comparable.