CDL - SECTION 6- COMBINATION
VEHICLES TEST
After coupling, how much space should be between the upper and lower fifth wheel? -
ANSWER-None
Describe what the service line is for. - ANSWER-Carries air, which is controlled by the foot
brake or the trailer hand brake.
Describe what the trailer air supply control does. - ANSWER-Push in -- Supply air to trailer;
Push out -- Shut air off and put on trailer emergency brakes
How can you test that air flows to all trailers? - ANSWER-Use parking brake and/or chock
wheels. Wait for normal air pressure, then push "trailer air supply" knob.
How can you test the tractor protection valve? - ANSWER-Charge the trailer air brake
system. Shut off engine and step on and off the brake pedal several times
How can you test the trailer emergency brakes? - ANSWER-Charge the trailer air brake
system and check that the trailer rolls freely. Stop and pull out the trail air supply control or
place in "emergency" position. Pull gently on the trailer with tractor to check emergency
brakes.
How can you test the trailer service brakes? - ANSWER-Check for normal air pressure.
Release the parking brakes. Move the vehicle forward slowly. Apply trailer brakes with the
band control.
Should you look into the back of the fifth wheel to see if it is locked onto the kingpin? -
ANSWER-Yes.
To drive, do you need to raise the landing gear only until it just lifts off the pavement? -
ANSWER-No, raise the landing gear all the way up.
What is a converter dolly? - ANSWER-a small trailer that can be coupled to a truck or trailer
so as to support a semi-trailer.
What is off-tracking? - ANSWER-When a vehicle goes around a corner and the rear
wheels follow a different path than the front wheels.
What is the emergency air line for? - ANSWER-Supplies air to the trailer air tanks; Controls
the emergency brakes on combination vehicles.
What might happen if the trailer is too high when you try to couple? - ANSWER-It may not
couple correctly.
