Air Leak and Late Low-Air Warning at Pre-Trip
During a pre-trip inspection at 5:45 a.m., you build air pressure on a 40-foot passenger coach until the governor cuts out at 122 psi. You shut off the engine and wait one minute. The gauge reads 117 psi; the previous minute it read 121 psi. You then press and hold the service brake pedal. After one minute the gauge has fallen from 117 psi to 110 psi. The low-air warning buzzer does not sound when you slowly drain the system; it finally sounds at 52 psi, and the spring brakes set at 32 psi. The bus is scheduled for a 6:00 a.m. school trip with 44 students. The shop is 25 miles away. There is no spare bus at the terminal. What do you do?
1. Interpret the leak-down and warning tests. Is the bus safe to dispatch?
Read the worked answer
No. With the engine off and the system fully charged, a single vehicle should lose no more than 3 psi in one minute; this bus lost 5 psi, from 122 to 117. With the service brake applied and held, the limit is 4 psi per minute; it lost 7 psi, from 117 to 110. Both readings indicate an air leak. The low-air warning should activate before pressure drops below 60 psi; this one first sounded at 52 psi, which is too late to give a driver time to react. Spring brakes setting at 32 psi is within the normal 20 to 45 psi range, but that does not cure the leak or the late warning. The bus is out of service until the leak is repaired and the warning is adjusted.
2. What do you do about the 6:00 a.m. school trip and the 25-mile distance to the shop?
Read the worked answer
Do not carry students in this bus, and do not drive it 25 miles to the shop on public roads unless a qualified mechanic authorizes a safe movement with no passengers. The defects affect the air brake system, so the risk is loss of braking, not just a warning light. Notify dispatch immediately, request a replacement bus, and document the defects on the driver vehicle inspection report. If no replacement is available, the trip waits or is cancelled; the schedule does not override an out-of-service condition. Have the shop repair the leak and verify governor cut-in and cut-out, static leak rate, applied leak rate, low-air warning before 60 psi, and spring brake application.