Vermont CDL Permit Test - Combination

If you do not know whether or not a trailer is equipped with ABS, look under the vehicle for the electronic control unit and wheel speed sensor wires coming from the back of the brakes.

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.

The trailer hand valve (also referred to as the trolley valve or Johnson bar) is a mechanism that works the trailer brakes. It should only be used to test the brakes. Using it while the vehicle is being driven could cause a skid. Never use the trailer hand valve while parking because doing so may release the air pressure from the braking system, releasing the brakes that are holding the vehicle in 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.

Air lines are sometimes color-coded to help drivers avoid mistakes when coupling glad hands. Typically, blue is used for service lines and red is used for emergency lines.

Tractor protection controls in older vehicles may be operated by levers instead of knobs. If an air supply control is set in its "emergency" position, the air supply will be stopped and the trailer emergency brakes will be applied.

Bobtail tractors are tractors that are not attached to any semitrailers. When operating a bobtail, you should be aware that stopping can be difficult and that it will take a longer distance to come to a complete stop than a tractor attached to a loaded semitrailer.