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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.
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.
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.)
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.
What happens when the tractor parking valve closes?
Which of the following describes the trailer air supply control on newer vehicles?
The tractor parking valve will close automatically if air pressure is in what range?
Which of the following statements is true about the trailer hand valve?
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