The Air Brake Test That Almost Passed
You are running the air brake check on a three-axle highway coach before a 300 km trip with 48 passengers. Tanks are fully charged; the governor cuts out at 125 psi and cuts in at 100 psi. At fast idle, pressure rises from 85 to 100 psi in 62 seconds. With the engine off, parking brake released and no pedal applied, the gauge falls 6 psi in one minute. With the service brake fully applied and held, it falls 5 psi in one minute. Repeating the low-pressure check, the warning buzzer and light come on at 55 psi, and the spring parking brakes apply at 32 psi. The coach is a single unit, and your yard standard for a single unit is 3 psi per minute.
1. Evaluate the build-up and leak numbers against the standard. Pass or fail, and what does each number actually tell you?
Read the worked answer
Build-up, 85 to 100 psi in 62 seconds - fail. The standard for a single unit is 45 seconds at fast idle. Slow build-up means a weak compressor, a mis-set governor, or air escaping somewhere. Either way the system recovers too slowly for stop-and-go work.
Unapplied leak, 6 psi/min - fail. The limit is 3 psi/min. With no pedal applied, leakage should be negligible; 6 psi/min points to a leaking hose, fitting or valve.
Applied leak, 5 psi/min - fail. Same 3 psi/min limit while holding a full application. This test loads the service chambers, diaphragms and valves, so a leak here sits in the parts that must hold pressure when you are stopping.
Why the three numbers matter together: on a route with frequent stops, a leaking system plus a slow compressor loses pressure faster than it is replaced. When it falls to roughly 20 to 45 psi the spring brakes apply by themselves - mid-block, with passengers standing in the aisle. That is the event the test exists to predict.
2. The warning came on at 55 psi and the spring brakes applied at 32 psi. Which result is acceptable, which is not, and why does the difference matter?
Read the worked answer
Warning at 55 psi - fail. The buzzer and light must operate before pressure drops below 60 psi, leaving the driver enough air to stop safely. At 55 psi it is late, and part of the margin you are supposed to have is already gone.
Spring brakes at 32 psi - pass. Automatic application should occur between roughly 20 and 45 psi. At 32 psi the mechanism is behaving as designed.
The difference matters because these are two separate systems with two separate jobs. The warning buys the driver time to pull over; the spring brakes are the last-resort backstop. Passing one does not offset failing the other. A bus can have perfectly functioning spring brakes and still be unsafe to dispatch, because the spring brake only rescues you after the pressure has already been lost - and on a highway there may be nowhere to be rescued to.
3. The trip leaves in 40 minutes. What is your decision, and what are you permitted to do about these numbers yourself?
Read the worked answer
Decision: the coach does not go. Three air tests fail - build-up rate, unapplied leak and applied leak - and the warning device is late. Together they describe a system that cannot hold or recover pressure.
What you may do: look for obvious sources. A chafed hose, a fitting visibly leaking, water in the tanks (drain them and note what comes out), the compressor belt. That is diagnosis.
What you may not do: adjust the governor, turn up the regulator, tape a leaking line, or dispatch on the argument that the gauge held fine yesterday. Those are mechanic decisions and must be documented.
Then: notify dispatch, record the failure with the actual numbers on the trip inspection report, and request a replacement coach. If a spare is provided, run the entire test again on it - a fresh vehicle is not a tested vehicle, and the replacement is the one you will be driving.