Inspecting Air Brake Systems (Pre-Trip and In-Service Checks)
The pre-trip air brake check
- Drain the tanks - moisture and oil collect at the bottom of every reservoir, including the wet tank.
- Build pressure. With the engine running, the gauge should climb from 85 to 100 psi in about 45 seconds. The governor should cut out near 125 psi and cut in near 100.
- Static leak test, brakes released. Engine off, system fully charged, parking brake released: loss of no more than 2 psi per minute on a single vehicle, 3 psi on a combination.
- Applied leak test. Hold the service brake fully applied: no more than 3 psi/min single, 4 psi/min combination.
- Low-pressure warning. Engine off, key on, pump the pedal down. The light and buzzer must come on before 60 psi.
- Spring brake test. Keep pumping. The parking brake knob should pop out and the spring brakes apply between 20 and 45 psi.
Slack adjusters and push rods
With the brakes released, mark the push rod at the chamber, then apply the brake fully. On a standard-stroke clamp-type type 30 chamber the rod should not move more than 2 inches. More travel means the brakes are out of adjustment. Traps: a driver may not adjust slack adjusters - that is a qualified mechanic's job - and with the brakes released the push rod should sit at roughly 90 degrees to the slack adjuster.
In service
Glance at the gauge regularly, listen for leaks, and check for air loss and dragging brakes at every stop. A wheel that smells hot or a truck that pulls under braking is a truck to inspect before it moves again.