Air Brake Manual

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Pre-trip procedure for air combination unit

Park the vehicle on level ground with the park brake set, the wheels blocked and the air tanks drained (if possible):

  1. Check security and condition of compressor, belts and air lines under hood.
  2. Start engine and let air pressure build up.
  3. With wheels blocked, release park brakes.
  4. Check brake adjustments (push rod travel) manually. Adjust if necessary.
  5. Verbally explain the proper procedure for adjusting an automatic slack adjuster.
  6. Governor operation (be sure spring brakes are released):
    • cut-out pressure between 120 and 145 psi (828 and 1,000 kPa)
    • cut-in pressure; fan brakes until compressor cuts in, usually about 20 to 25 psi (138 to 172 kPa) less than cut-out pressure, but at a minimum of 100 psi (690 kPa)
  7. Charge trailer system and rebuild pressure. Shut off engine.
  8. Break service line (no air loss should occur).
  9. Break supply line:
    • trailer brakes should apply immediately
    • there should be no air loss from trailer line
    • air from truck should shut off at a minimum pressure of 20 psi (138 kPa)
  10. Make and hold a foot brake application:
    • check gauge for any air loss after initial application
    • listen for audible air leaks
    • there should be no air loss from the service line
  11. Reconnect lines, charge trailer and rebuild pressure.
  12. At maximum pressure:
    • release park brake
    • shut off engine
  13. Make and hold full foot-brake application:
    • maximum air loss after initial application is four psi (28 kPa) in one minute
    • listen for audible air leaks
  14. With ignition key on, fan brakes to lower air pressure:
    • low warning system should operate at a minimum of 60 psi (414 kPa)
    • continue to fan brakes, trailer-supply valve should shut off air to trailer at a minimum of 20 psi (138 kPa)
    • truck park brake valve may shut off although on some vehicles the button may never close. Always ensure the spring brakes have been fully applied. On some vehicles the button may never close, however always ensure the spring brakes have been fully applied
  15. Run the engine between 600 and 800 rpm and observe the time needed for air pressure to rise from 85 to 100 psi (586 to 690 kPa) on the truck only. It should be less than two minutes.
  16. Ensure park brakes are applied, remove wheel blocks.
  17. Four final tests:
    • with trailer emergency brakes applied and truck park brakes released, try to gently pull ahead to test emergency application of trailer brakes
    • charge trailer, apply park brakes on the truck only and try to gently pull ahead
    • release park brakes, move slowly ahead and apply trailer brakes with hand valve, if equipped
    • move slowly ahead and make foot-brake application

Note: Repeat hand and foot-valve test on both sides of the unit checking for response and, in winter, for frozen wheels.

Build-up time procedure
Do not charge trailer. Begin timing when the primary needle on the dash gauge reaches 85 psi (586 kpa) and stop timing when the same needle reaches 100 psi (690 kpa).


Rev: 2018