MVA CDL Combination Vehicles Study
Guide with Complete Solutions
After coupling, how much space should be between the upper and lower fifth wheel? -
ANSWER-There should be no space between the upper and lower fifth wheel.
Describe what the service line is for. - ANSWER-(Also called the control line or signal line)
Carries air, which is controlled by the foot brake or trailer hand brake. Depending on how
hard you press the foot brake or hand valve, the pressure in the service line will similarly
change.
Describe what the trailer air supply control does. - ANSWER-You push it in to supply the
trailer brakes with air, and pull it out to shut off the air supply to put the trailer emergency
brakes on.
How can you test that air flows to all trailers? - ANSWER-At the rear of the rig, Open the
emergency line shut-off valve at the rear of the last trailer. You should hear air escaping,
showing the entire system is charged, close the shut-off; Open the service line valve to
check that service pressure goes through all the trailers.
How can you test the tractor protection valve? - ANSWER-With the air-brake system
charged, turn off engine, pump brake pedal to fan out air, the trailer air supply control (aka
tractor protection valve) should pop out when the air pressure falls between 20-45 psi
How can you test the trailer emergency brakes? - ANSWER-Charge the trailer air brake
system and check the trailer moves freely, pull trailer air supply control, pull gently on the
trailer with the tractor to check that the trailer emergency brakes are on
How can you test the trailer service brakes? - ANSWER-Check for normal air pressure,
release parking brakes, move the vehicle, apply trailer brakes with the hand control, you
should feel the brakes come on showing the service brakes are working
How do you know if your trailer is equipped with antilock brakes? - ANSWER-Trailers will
have yellow ABS malfunction lamps on the left side, either on front or rear corner.
To drive, raise the landing gear _____________________. - ANSWER-All the way up
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-It supplies air to the trailer air tanks;
controls the emergency brakes on combination vehicles.
