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6.2 – Combination Vehicle Air Brakes

Before reading this section, you should study Section 5: Air Brakes. In combination vehicles, the braking system has parts to control the trailer brakes, in addition to the parts described in Section 5. These parts are described below.

6.2.1 – Trailer Hand Valve

The trailer hand valve (also called the trolley valve or Johnson bar) works the trailer brakes. The trailer hand valve should be used only to test the trailer brakes. Do not use it in driving because of the danger of making the trailer skid. The foot brake sends air to all the brakes on the vehicle, including the trailer(s). There is much less danger of causing a skid or jackknife when using just the foot brake.

Never use the hand valve for parking because all the air might leak out, unlocking the brakes (in trailers that do not have spring brakes). Always use the parking brakes when parking. If the trailer does not have spring brakes, use wheel chocks to keep the trailer from moving.

6.2.2 – Tractor Parking Valve

The tractor parking valve keeps air in the tractor or truck brake system should the trailer break away or develop a bad leak. The tractor parking valve is controlled by the “trailer air supply” control valve in the cab. The control valve allows you to open and shut the tractor parking valve. The tractor parking valve will close automatically if air pressure is low (in the range of 20 to 45 psi). When the tractor parking valve closes, it stops any air from going out of the tractor. It also lets the air out of the trailer emergency line. This causes the trailer emergency brakes to come on, with possible loss of control. (Emergency brakes are covered later.)

6.2.3 – Trailer Air Supply Control

The trailer air supply control on newer vehicles is a red eight-sided knob, which you use to control the tractor parking valve. You push it in to supply the trailer with air and pull it out to shut the air off and put on the trailer emergency brakes. The valve will pop out (thus closing the tractor parking valve) when the air pressure drops into the range of 20 to 45 psi.

Tractor parking valve controls or “emergency” valves on older vehicles may not operate automatically. There may be a lever rather than a knob. The “normal” position is used for pulling a trailer. The “emergency” position is used to shut the air off and put on the trailer emergency brakes.

Multiple-Choice Questions:

Question #323 (1 of 4)

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What happens when the tractor parking valve closes?

  • None of these answers are correct
  • It lets air out of the tractor and into the trailer emergency line. This applies the trailer brakes.
  • It keeps air in the trailer emergency line
  • It stops any air from going out of the tractor. It also lets the air out of the trailer emergency line.
When the tractor parking valve closes, it stops any air from going out of the tractor. It also lets the air out of the trailer emergency line.
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Question #326 (2 of 4)

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Which of the following describes the trailer air supply control on newer vehicles?

  • It's a red eight-sided knob that controls the trailer parking valve. You pull it out to supply the trailer with air and push it in to shut off the air and put on the trailer emergency brakes
  • It's a red eight-sided knob that controls the tractor parking valve. You push it in to supply the trailer with air and pull it out to shut off the air and put on the trailer emergency brakes
  • It's a yellow round knob that controls the trailer brakes. You push it in to supply the trailer with air and pull it out to shut off the air and put on the trailer emergency brakes
  • It's a yellow round knob that controls the tractor brakes. You push it in to supply the tractor brakes with air and pull it out to shut off air to the tractor brakes
The trailer air supply control on newer vehicles is a red eight-sided knob, which you use to control the tractor parking valve. You push it in to supply the trailer with air and pull it out to shut the air off and put on the trailer emergency brakes.
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Question #325 (3 of 4)

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The tractor parking valve will close automatically if air pressure is in what range?

  • 40 to 60 psi
  • 0 to 20 psi
  • 20 to 45 psi
  • 60 to 80 psi
The tractor parking valve will close automatically if air pressure is low (in the range of 20 to 45 psi)
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Question #324 (4 of 4)

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Which of the following statements is true about the trailer hand valve?

  • It works the trailer brakes. It should be used only to test the trailer brakes. Do not use it in driving because of the danger of making the trailer skid.
  • It works the trailer brakes. It's main function is to give the driver fine control over the trailer brakes for better maneuvering in slick conditions. It can also be used as a parking brake.
  • It works the trailer brakes. It serves mostly as a parking brake, especially when you're parked on a hill. It can be used under normal driving circumstances when only slight braking is needed.
  • It works the trailer brakes. It can be used for testing the trailer brakes or for stopping the trailer under normal conditions to save wear on the tractor brakes
The trailer hand valve (also called the trolley valve or Johnson bar) works the trailer brakes. The trailer hand valve should be used only to test the trailer brakes. Do not use it in driving because of the danger of making the trailer skid.
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