The 8 psi/min Bus at the Curb
It is 06:05 and you are assigned a 40-foot transit bus on a 42-minute airport run with 38 passengers already gathering at the curb. During your pre-trip, the air gauge reads 120 psi. With the engine off, wheels chocked, parking brake released, and the service brake held fully applied for two minutes, pressure falls from 120 to 104 psi and is still creeping down. The compressor builds pressure normally at high idle, the low-pressure warning buzzer sounds at 58 psi, and the spring brakes set when you fan the pressure down. The shop is 0.3 miles away with one mechanic on duty; the relief bus is 35 minutes out, and dispatch wants you to roll and swap at the airport.
1. Name the test you are actually performing, show the arithmetic, and make the go/no-go call.
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The test: engine off, system fully charged, wheels chocked, parking brake released, service brake held fully applied - this is the static leak test with brakes applied. For a single vehicle (no trailer) the limit is 4 psi per minute. The 3 psi/min figure belongs to the same test with the brakes released.
The arithmetic: 120 − 104 = 16 psi lost in 2 minutes = 8 psi/min, double the allowable rate.
What does not rescue it: the compressor building normally only proves the compressor works - the leak is downstream of it. The buzzer sounding at 58 psi proves the warning circuit works; it does not slow the leak. The spring brakes setting during the fan-down is a normal check, not a pass on leakage.
Call: no-go. A leak of this size is an out-of-service defect, not a squawk sheet item. The bus does not move with passengers aboard.
2. Dispatch says: "It's been like that for weeks - take it and swap at the airport." Write your reply, with numbers.
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Refuse, and give the reason in numbers rather than opinion.
Starting at 120 psi and losing 8 psi/min, the gauge reaches the 60 psi warning in about 7.5 minutes of continuous brake application. Spring brakes begin to drag in roughly the 20–45 psi band. On a 42-minute run with 38 passengers, a long light, a grade, or heavy traffic can eat that margin - the failure mode is not "a soft pedal," it is a full loss of service brakes with a loaded bus.
"It's been like that for weeks" describes a known, unrepaired defect. That makes it worse, not better: it is now a documented condition you knew about.
Do it in writing: text or dispatch-log entry stating the test performed, the 120→104 psi reading, the 8 psi/min rate, and the refusal. Red-tag the bus.
3. What happens to the 38 passengers, and may you run the bus the 0.3 miles to the shop yourself?
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Passengers: they do not board. Keep them inside the terminal or behind the curb line, not clustered around a bus that is not going anywhere. Announce the relief bus and the 35-minute delay once, clearly, and stay visible - the second-most common failure at this point is leaving a crowd uninformed and angry at the curb.
The 0.3 miles: no. Distance does not change the defect, and 0.3 miles at 06:05 is a city block and a half of intersections - exactly where you need brakes. With passengers aboard, it is a hard no.
If it must be repositioned at all, it goes empty, at walking pace, driven by maintenance, after the leak is repaired or the affected component is capped. The default answer on a leak-rate failure is: fixed in place, or towed.