Montana CDL Permit Test - Combination

The trailer hand valve (also referred to as the "trolley valve" or "Johnson bar") works the trailer brakes and should be used only when testing them. Using the trailer hand valve while driving could result in a skid.

The emergency air line (also known as the supply line) has two functions. The first is to supply air to the trailer air tanks on a combination vehicle, and the second is to control the emergency brakes.

If a trailer begins to jackknife, you should release the brakes and allow the trailer wheels to regain traction with the road. Do not apply the trailer hand brake to attempt to straighten out the rig because it will only prolong the skid. The application of brakes is what caused the wheels to lock in the first place.

When operating a vehicle with an Anti-Lock Braking System (ABS), you should apply your brakes in the same manner as you would if operating a vehicle without ABS.

Tractor protection controls in older vehicles may be operated by levers instead of knobs. If an air supply control is set in its "normal" position, it is properly set for you to pull a trailer.

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.

When making a turn, be sure to steer gently and smoothly. Making a quick turn while pulling trailers is dangerous and can increase the risk of the last trailer rolling over.