How to Hook Up the Air Supply on a Tractor Trailer

by Jeff Miller
line of trucks 1 image by Aaron Kohr from

Of all of the systems on a tractor trailer, the air supply system is the most important because it controls the rig's brakes. Every time a trailer is hooked to a tractor the air supply system must be hooked up by the operator.

Step 1

Look for the air line couplers on the tractor and trailer. Two air lines extend from the back of the tractor -- one is red and one is blue. Each line has a coupler on the end of it called a glad hand that mates with a corresponding glad hand on the front of the trailer.

Step 2

Engage the brakes by pulling out the red brake knob on the dashboard of the tractor. The yellow knob next to it should pop out automatically. If it doesn't, pull it out manually.

Step 3

Check the glad hand seals on the trailer and the tractor. These are rubber grommets seated inside of the glad hands. If any are cracked or missing, they must be replaced immediately.

Step 4

Connect the service line -- the blue one - to the blue glad hand on the trailer by positioning the faces of the glad hands together and turning them clockwise until the grooves lock. Repeat this operation for the emergency air line -- the red one.

Step 5

Couple the tractor to the trailer.

Step 6

Engage the brakes by pulling out the red brake knob, then shift into first gear and slowly ease the tractor forward. This is called a pull test. The trailer should restrain the tractor from moving forward because the brakes are engaged. If the trailer moves forward during the pull test, the trailer brakes need to be serviced before the vehicle is operated.

Disengage the brakes by pushing the red and yellow brake knobs in on the dashboard. Watch the pressure gauge. It should equalize between 70 and 120 psi within a minute, then maintain its pressure. The air supply system is now properly hooked up.


  • If you accidentally hook the air lines up backward, the trailer will not move, even with the brake knobs in the disengaged position. One way to tell that this is the case is to listen for a large amount of air escaping from behind the red and yellow brake knobs on the dashboard. Reversing the air lines will remedy this problem.


  • The brakes should be engaged any time you leave the cab of the tractor. This will prevent the tractor from rolling. Chock the wheels of the tractor when not inside the cab.
  • Always conduct a complete pre-trip inspection every day before the tractor-trailer is placed into service.

Items you will need

  • Leather work gloves

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