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Combination Vehicles Free CDL Practice Tests
Page 4

Prepare For The Combination Vehicles Portion Of Your CDL Written Exams

Combination Vehicles Questions

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Supplied by the service line from the tractor, each trailer or dolly has its own:
  • Brake pedal
  • Steering wheel
  • Air tanks
  • Air compressor
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From The CDL Manual

Trailer Air Tanks —

Each trailer and converter dolly has one or more air tanks. They are filled by the emergency (supply) line from the tractor. They provide the air pressure used to operate trailer brakes. Air pressure is sent from the air tanks to the brakes by relay valves. The pressure in the service line tells how much pressure the relay valves should send to the trailer brakes. The pressure in the service line is controlled by the brake pedal and the trailer hand brake.

Next
The shut-off valves at the back of the last trailer should be:
  • Closed
  • Open
  • Removed
  • Covered
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From The CDL Manual

Shut-off Valves—

Shut-off valves (also called “cut-out cocks”) are used in the service and supply air lines at the back of trailers used to tow other trailers. These valves permit closing the air lines off when another trailer is not being towed. You must check that all shut-off valves are in the open position except the ones at the back of the last trailer, which must be closed.

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Next
At the back of trailer used to tow other trailers are shut-off valves, also called:
  • Shut-off knobs
  • Cut-out knobs
  • Cut-out cocks
  • Shut-off cocks
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From The CDL Manual

Shut-off Valves—

Shut-off valves (also called “cut-out cocks”) are used in the service and supply air lines at the back of trailers used to tow other trailers. These valves permit closing the air lines off when another trailer is not being towed. You must check that all shut-off valves are in the open position except the ones at the back of the last trailer, which must be closed.

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Next
When parking trailers without spring brakes, it is very important to:
  • Never disconnect the trailer
  • Let your dispatcher know
  • Use wheel chocks
  • Park against the curb
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From The CDL Manual

The emergency brakes come on whenever the air supply knob is pulled out or the trailer is disconnected. But the brakes will hold only as long as there is air pressure in the trailer air tank. Eventually, the air will leak away, and then there will be no brakes. Therefore, it is very important for safety that you use wheel chocks when you park trailers without spring brakes.

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Next
Spring brakes are not required on trailers and dollies built before:
  • The tractor
  • 1975
  • 1995
  • 1982
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From The CDL Manual

Trailer Service, Parking and Emergency Brakes —

Newer trailers have spring brakes just like trucks and truck tractors. However, converter dollies and trailers built before 1975 are not required to have spring brakes. Those that do not have spring brakes have emergency brakes that work from the air stored in the trailer air tank.

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Next
A major leak in the trailer emergency line will cause:
  • An air leak could cause all of these things
  • A quick loss of air pressure
  • The trailer emergency brakes to come on
  • The tractor protection valve to close
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From The CDL Manual

A major leak in the emergency line will cause the tractor protection valve to close and the trailer emergency brakes to come on.

You may not notice a major leak in the service line until you try to put the brakes on. Then, the air loss from the leak will lower the air tank pressure quickly. If it goes low enough, the trailer emergency brakes will come on.

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Next
A fifth wheel in proper position for coupling will have:
  • All of these things
  • Jaws open
  • Safety unlocking handle in the automatic lock position
  • Wheel tilted down towards rear of tractor
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From The CDL Manual

Check if fifth wheel is in proper position for coupling:

  • Wheel tilted down towards rear of tractor.
  • Jaws open.
  • Safety unlocking handle in the automatic lock position.
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Next
Improperly greased or lubricated fifth wheels will cause:
  • Braking problems
  • Air pressure problems
  • Steering problems
  • No problems
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From The CDL Manual

Be sure that the fifth wheel plate is greased as required. Failure to keep the fifth wheel plate lubricated could cause steering problems because of friction between the tractor and trailer.

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Next
When coupling or uncoupling, all of these apply to the trailer except:
  • Make sure cargo is secured
  • Make sure area around the vehicle is clear
  • Chock the wheels or apply the spring brakes
  • Make sure trailer doors are open
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From The CDL Manual

STEP 2: Inspect Area and Chock Wheels

  • Make sure area around the vehicle is clear.
  • Be sure trailer wheels are chocked or spring brakes are on.
  • Check that cargo (if any) is secured against movement due to tractor being coupled to the trailer.
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Next
The tractor should be positioned to back under the trailer:
  • At a 45 degree angle
  • Straight back
  • At a 90 degree angle
  • In whatever position is convenient
Click here to look up the answer

From The CDL Manual

STEP 3: Position Tractor

Put the tractor directly in front of the trailer. (Never back under the trailer at an angle because you might push the trailer sideways and break the landing gear.)

Check position, using outside mirrors, by looking down both sides of the trailer.

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