Pre-Trip Inspection and Leak Testing
A sequence that works
Chock the wheels, drain all reservoirs and watch for moisture or oil, check the compressor belt, oil level and obvious damage, then start the engine and let pressure build at fast idle. Note the pressure where the governor cuts out. Switch the engine off, release the parking brake and let the system stabilise, then run the two leak tests: with brakes released, watch the gauge for one minute; then hold a full service brake application for one minute and watch again. Common limits are no more than about 2 to 3 psi per minute released, and 3 to 4 psi per minute applied, with tighter figures for combinations - always use your provincial manual.
Warning devices and protection valves
Fan the brakes down and confirm the low-pressure warning buzzer and light come on before pressure drops to about 60 psi. Keep releasing air and watch the spring brakes apply as the push-pull valve pops out, usually somewhere around 20 to 45 psi. Then test the tractor protection valve by charging the trailer, shutting off the engine and fanning down; the valve should close and protect the tractor. Check glad hand seals, hoses for chafing, slack adjuster and pushrod travel against the limit for the chamber size, drum or rotor condition, and that the parking brake holds.
Traps
Running the leak test with the engine idling, or with the parking brake still applied. Not fanning down to reach the warning threshold. Pulling the yellow knob instead of pushing it, or mistaking the red trailer supply valve for the parking control. Ignoring a slightly long pushrod stroke.
Worked example
Gauge reads 100 psi, engine off, brakes released. After one minute it reads 92 psi. That is 8 psi lost - several times a normal limit - so the vehicle is unsafe and must be repaired before use.