Inspecting the Air Brake System (Pre-Trip Checks)
A repeatable sequence
Use the same order every time so nothing is skipped. Start with the engine off and chock the wheels.
- Drain the tanks. Open drain cocks on supply, primary, and secondary tanks and let moisture and oil out; close them. On a vehicle with an air dryer, some moisture is normal but milky or oily discharge is not.
- Build pressure. Run the engine at fast idle and watch the gauge climb; confirm the governor cuts out near 125 psi and the compressor cut-in near 100 psi.
- Warning devices. With the engine off, pump the brake pedal to reduce pressure; the low-pressure buzzer/light must come on before 60 psi. On vehicles with a spring brake, the parking brake knob should pop out (apply) as pressure falls, typically in the 20–45 psi range.
- Static leak test. With a full system, engine off, brakes released, watch the gauge for one minute; then hold the pedal and watch again. Compare to the limits for your vehicle (commonly under 3 psi released, under 4 psi applied for a single vehicle).
- Applied leak and stroke check. With the parking brake released and wheels chocked, have a helper hold a full brake application while you check for audible leaks and measure pushrod stroke against the slack adjuster limit. Also check that the spring brake releases and applies with the knob.
Traps and a rule of thumb
Examiners ask when to check something: the warning test happens after pumping pressure down with the engine off, not while the compressor is running. Another trap is measuring stroke with the parking brake set - you cannot, because the spring brake is applied. Never crawl under a vehicle to listen for leaks; use a helper or listen from outside the wheel area.
Rule of thumb: drain, build, warn, leak, stroke - in that order.