Maine CDL Permit Test - Combination

Before a trip, be sure to inspect all couplings. There should be no space between the upper and lower fifth wheel.

Before a trip, you should ensure that air reaches all air brakes on all trailers by opening up the rear emergency line and service line shut-off valves and listening for escaping air. Close both shut-off valves before beginning to drive.

When connecting a converter dolly to a second or third trailer, the trailer height should correct. It should be slightly lower than the center of the fifth wheel.

Shut-off valves are used in air supply lines to control the passage of air from one trailer to another. The rear shut-off valve should always be closed to prevent air from leaving the braking system.

Be sure to test the trailer emergency brakes before beginning a trip. After ensuring that the trailer rolls freely, you can test the emergency brakes by pulling out the trailer air supply control, or placing it in the "emergency" position. Pull forward slightly with the tractor and make sure the trailer does not move.

If your trailer begins to skid, you should release the brakes to regain traction. The trailer will begin to straighten out once the wheels begin to grip the road again.

If you are operating a vehicle built before 1998 and are unsure if the trailer has an Anti-Lock Braking System (ABS), you can check under the trailer for wheel speed sensors coming from the back of the brakes.