Inspecting and Maintaining Air Brake Systems
This domain covers the inspection sequence an examiner will watch you perform, plus the numbers and wear limits behind each step.
Order of the air brake check
- Run the engine to governor cut-out (about 125 psi), then shut it off.
- Release the parking brake and watch the gauge for one minute: static leak no more than 3 psi on a single vehicle, 4 psi on a combination.
- Hold the service brake fully applied for one minute: applied leak no more than 4 psi single, 6 psi combination.
- Turn the key on and fan off air with the pedal. The warning light or buzzer must come on before 60 psi.
- Keep fanning. The spring brakes must set by 45 psi (they apply in the 20–45 range).
- Restart. Pressure should climb from 85 to 100 psi in 45 seconds or less at operating RPM, and the governor should cut out near 125.
Push rods and slack adjusters
- With the brakes released, pull each push rod by hand: more than about 1 inch of movement means it is out of adjustment. With the brakes applied, travel must stay inside the limit marked for that chamber; long-stroke chambers allow more.
- Automatic slack adjusters still get measured. If one keeps going out, find the cause instead of repeatedly adjusting it.
Moisture control
- Drain every reservoir daily, wet tank first, and more often in humid or freezing weather. Service the dryer filter and check the alcohol evaporator.
Worked example
A combination vehicle loses 4 psi in one minute with the parking brake released and the engine off. That passes for a combination (limit 4 psi) but the identical reading on a single vehicle would fail (limit 3 psi). Always know which vehicle you are testing.
Traps
- Running leak tests with the engine on or the parking brake set.
- Mixing up 3/4 psi (static) with 4/6 psi (applied).
- Pumping the pedal instead of holding it.
- Adjusting a slack adjuster to hide a leaking chamber or worn lining.