Arkansas CDL Permit Test - Combination

The trailer hand valve, also called the trolley valve or Johnson bar, is used to work the trailer brakes. It should be used only to test the brakes. Using it while a vehicle is in motion could cause a skid.

You should test the tractor protection valve by allowing the air supply to properly charge, turning off the engine, then releasing air from the system by pressing the brake pedal several times. The tractor protection valve should move into its emergency position when psi drops to an unsafe level.

A tractor protection valve keeps air in a tractor or truck air brake system should the trailer break away or develop a bad leak. The valve will close automatically if the pressure drops to an unsafe level.

Push in the trailer air supply control to supply the trailer with air. Pull out the trailer air supply control to turn off the air supply and activate the emergency brakes.

When driving a combination vehicle, it is important to brake well in advance and look far ahead of your vehicle to avoid the need for sudden stops. Making habits of these actions will help you prevent your trailer from jackknifing.

When driving a tractor-trailer combination that is equipped with an Anti-Lock Braking System (ABS), you should brake in the same manner that you would if the vehicle was not equipped with ABS.

When a vehicle goes around a corner, the rear wheels follow a different path than the front wheels. This is known as off-tracking, or "cheating."