Parts and Service >> Air Brakes on Combination Vehicles

• The trailer hand valve, also called the "trolley valve" or "Johnson Bar", works the trailer brakes.
  • ONLY use the trailer hand valve to test the trailer brakes.
  • Do NOT use the trailer hand valve when driving.
  • Do NOT use the trailer hand valve for a parking brake.

• The tractor protection valve closes automatically if there is a sudden trailer leak or low pressure. This stops the air from going out of the tractor.

• Trailer Air Supply Control
  • In new trucks; a red 8-sided knob is pushed in to supply the trailer with air. The knob is pulled out to put on the emergency brakes. If the air pressure drops too low, the trailer air supply control will automatically put on the emergency brake.

  • In older trucks; the trailer air supply control may be a lever or a knob. It is operated by hand to put on the lever or a knob. It is operated by hand to put on the emergency brake. The "EMERGENCY" position is used to shut the air off and put on the trailer emergency brakes.

• Trailer Air Lines
  • Service Air Line
    1. The service line is also called the "control line".
    2. The service line carries air, which is controlled by the foot brake or the trailer hand brake.
    3. The service line is connected to relay valve(s).
    4. The service line is usually colored blue.

  • Emergency Line
    1. The emergency line is also called the "supply line".
    2. The emergency line supplies air to the trailer air tanks.
    3. The emergency line controls the emergency brakes.
    4. The emergency line is colored red.

• Glad Hands
  • Glad hands are used to connect the service and emergency air lines from the truck or tractor to the trailer.
  • Clean the glad hands and rubber seals before a connection is made.
  • Attach unused hoses to "dead end" or dummy couplers. This keeps water and dirt from getting into the coupler and the air lines.
  • If you cross the air lines, supply air will be sent to the service line instead of going to charge the trailer air tanks. If the spring brakes do not release when you push the trailer air supply control, check the air line connections.
  • On trailers without spring brakes, if the air lines were crossed, you can drive but will not have trailer brakes. Test the trailer brakes before driving, using the hand valve or by pulling the air supply control.

• Trailer Air Tanks
  • Each trailer and converter dolly has one or more air tanks.
  • Air tanks are filled by the emergency supply line from the tractor.
  • Air pressure is sent from the air tanks to the brakes by relay valves.
  • The pressure in the service line tells how much pressure the relay valves should send to the trailer brakes.
  • Do not let water and oil build up in the air tanks. Drain air tanks of water and oil after each working day. Water can freeze in parts of the brake system making the brakes fail.

• Trailer Service, Parking and Emergency Brakes
  • On trailers without spring brakes, the emergency brakes come on whenever the air pressure in the emergency line is lost.
  • These trailers have no parking brake.
  • The emergency brakes will hold only as long as there is air pressure in the trailer air tank.
  • It is important for safety that you use wheel chocks when you park trailers without spring brakes.
  • A major leak in the service line will lower the air tank pressure quickly and cause the trailer emergency brakes to come on.