CDL Combination Vehicles Practice Test 2026: Practice test 3

1. Question 1 of 20

   After supplying air to the trailer, how can you check that the air lines are not crossed?

   • ACheck the engine oil pressure gauge for a change
   • BDrive at 30 mph and brake hard to see what happens
   • CApply and release the trailer brakes and listen for them
   • DLook only at the colors of the two air lines
   Show answer

   Answer: C. Apply and release the trailer brakes and listen for them

   Apply and release the trailer brakes and listen for them working. If they don't, the lines may be crossed.

2. Question 2 of 20

   Before backing under the trailer, what should you do with the trailer brakes?

   • ADisconnect the air lines so they stay released
   • BPump them several times with the hand valve
   • CLock them by pulling out the air supply knob
   • DRelease them by pushing in the air supply knob
   Show answer

   Answer: C. Lock them by pulling out the air supply knob

   Lock the trailer brakes by pulling the air supply knob (or moving the valve to emergency) before backing under.

3. Question 3 of 20

   The diagram shows the tractor backing under the trailer. Which gear should you use?

   • ANeutral
   • BFirst forward gear
   • CThe highest reverse gear
   • DThe lowest reverse gear
   Show answer

   Answer: D. The lowest reverse gear

   Back slowly in the lowest reverse gear so you do not hit the kingpin too hard. Stop when it locks.

4. Question 4 of 20

   How do you test that the trailer is locked onto the tractor?

   • ARelease the trailer brakes and back up hard
   • BPull gently forward with the trailer brakes locked
   • CShake the trailer by hand from the side
   • DDrive away at normal speed and watch the mirrors
   Show answer

   Answer: B. Pull gently forward with the trailer brakes locked

   This tug test, with the trailer brakes still locked, shows whether the kingpin is locked in the jaws.

5. Question 5 of 20

   When you inspect the coupling, what should you see between the upper and lower fifth wheel?

   • AAbout one inch of space
   • BNo space at all
   • CGrease dripping out
   • DA gap large enough to see light
   Show answer

   Answer: B. No space at all

   There should be no space. A gap means the kingpin may be resting on the jaws, and the trailer could come loose.

6. Question 6 of 20

   Where should the fifth wheel's locking jaws be closed?

   • AAround the shank of the kingpin
   • BAround the head of the kingpin
   • CAround the glad hands
   • DAround the landing gear
   Show answer

   Answer: A. Around the shank of the kingpin

   Look into the back of the fifth wheel to make sure the jaws are closed around the shank, not the head, of the kingpin.

7. Question 7 of 20

   After coupling, where should the fifth wheel locking lever be?

   • AIn the open position, ready for uncoupling
   • BRemoved and stored in the cab while driving
   • CIn the lock position, with the safety latch over it
   • DHalfway between open and lock, for easy release
   Show answer

   Answer: C. In the lock position, with the safety latch over it

   The locking lever must be in the lock position and the safety catch must be in place.

8. Question 8 of 20

   How should you raise the landing gear after coupling?

   • AUse high range only, to save time
   • BLeave it down if the trip is short
   • CRaise it halfway so it is ready to lower
   • DLow range first, then high range, all the way up
   Show answer

   Answer: D. Low range first, then high range, all the way up

   Lift the weight off in low range, then use high range. Never drive with the landing gear partly raised. Secure the crank handle.

9. Question 9 of 20

   After the landing gear is up, what clearance should you check?

   • ATractor frame to landing gear, and tractor tires to trailer nose, in turns
   • BMud flaps to the road surface, only when the trailer is empty
   • CTrailer roof to low branches, only on the first trip of the day
   • DCab to sleeper berth, only when the fifth wheel is moved
   Show answer

   Answer: A. Tractor frame to landing gear, and tractor tires to trailer nose, in turns

   With the trailer's full weight on the tractor, make sure there is room in sharp turns.

10. Question 10 of 20

    What is the final step of coupling a tractor to a semitrailer?

    • AConnect the service and emergency air lines
    • BRemove the wheel chocks and store them
    • CLock the trailer brakes with the air supply knob
    • DBack slowly under the trailer to lock the kingpin
    Show answer

    Answer: B. Remove the wheel chocks and store them

    After raising the landing gear, remove the trailer wheel chocks and store them safely.

11. Question 11 of 20

    Why must the fifth wheel plate be greased?

    • ALack of grease can cause steering problems
    • BGrease keeps the trailer lights working
    • CGrease is needed for the trailer brakes
    • DGrease makes the trailer weigh less
    Show answer

    Answer: A. Lack of grease can cause steering problems

    The fifth wheel plate needs enough grease. Without it, friction between tractor and trailer can cause steering problems.

12. Question 12 of 20

    This diagram shows a sliding fifth wheel. What must you check on it?

    • AThat the locking pins have been removed
    • BThat it is set as far forward as it will go
    • CThat the slide can move freely while driving
    • DThat all locking pins are present and locked
    Show answer

    Answer: D. That all locking pins are present and locked

    On a sliding fifth wheel, the slide must not be damaged, it must be greased, and all locking pins must be present and locked.

13. Question 13 of 20

    The diagram shows a sliding fifth wheel. Why must it not be set too far forward?

    • AThe trailer would ride too high to couple
    • BThe kingpin would no longer fit the jaws
    • CThe air lines would be stretched too long
    • DThe frame could hit the landing gear in turns
    Show answer

    Answer: D. The frame could hit the landing gear in turns

    Check that the fifth wheel is not so far forward that the frame hits the landing gear or the cab hits the trailer when turning.

14. Question 14 of 20

    What should you check on the air lines between the tractor and trailer?

    • APulled tight with no slack, so they can't drag
    • BTaped to the frame so they cannot move at all
    • CResting on the drive shaft so they stay warm
    • DConnected, not leaking, with enough slack for turns
    Show answer

    Answer: D. Connected, not leaking, with enough slack for turns

    Air lines must be properly connected with no leaks, and have enough slack for turns without dragging or rubbing.

15. Question 15 of 20

    What should you check on the landing gear before driving?

    • ALowered so it just touches the road
    • BCrank handle removed and stored in the cab
    • CHalfway down, ready for the next stop
    • DFully raised, undamaged, crank handle secured
    Show answer

    Answer: D. Fully raised, undamaged, crank handle secured

    The landing gear must be fully up with no missing or damaged parts, and the crank handle in place and secured.

16. Question 16 of 20

    How do you test the tractor protection valve?

    • ALower the pressure with the brake; the supply knob should pop out
    • BRun the engine at high rpm until the supply knob pops in
    • CPull the supply knob out while driving at highway speed
    • DDisconnect the service line while driving and watch the gauge
    Show answer

    Answer: A. Lower the pressure with the brake; the supply knob should pop out

    Lowering the air pressure should make the trailer air supply control pop out within the maker's range, usually 20 to 45 psi.

17. Question 17 of 20

    How do you test the trailer emergency brakes?

    • ADisconnect the trailer's electrical cord while parked
    • BPull out the trailer air supply control and pull gently forward
    • COpen the trailer tank drain valves and wait one minute
    • DPress the brake pedal hard while driving at highway speed
    Show answer

    Answer: B. Pull out the trailer air supply control and pull gently forward

    With the air supply control out (emergency), the trailer brakes should hold when you pull gently against them.

18. Question 18 of 20

    How do you test the trailer service brakes?

    • ABrake hard with the foot pedal while backing up
    • BMove forward slowly and apply the trailer hand valve
    • CPull out the red knob while driving at highway speed
    • DSet the yellow knob and try to drive away
    Show answer

    Answer: B. Move forward slowly and apply the trailer hand valve

    Creep forward and use the hand control. You should feel the trailer brakes come on.

19. Question 19 of 20

    Which CDL class covers a tractor-semitrailer with a GCWR of 80,000 pounds and a trailer rated over 10,000 pounds?

    • AClass B
    • BClass B with a T endorsement
    • CClass C
    • DClass A
    Show answer

    Answer: D. Class A

    Combinations of 26,001 pounds or more GCWR, towing a unit rated over 10,000 pounds, need a Class A CDL.

20. Question 20 of 20

    You take the Class A skills test in a combination joined by a pintle hook, not a fifth wheel. What restriction will you get?

    • ANo hauling of hazardous materials
    • BNo driving at night or in bad weather
    • CNo Class A tractor-trailers with a fifth wheel
    • DNo driving vehicles with air brakes
    Show answer

    Answer: C. No Class A tractor-trailers with a fifth wheel

    Testing with a pintle hook or other non-fifth-wheel connection brings a tractor-trailer restriction.

Keyboard: 1–4 to answer, Enter for the next question.

The real test: 80% correct; the number of questions varies by state (20 in Washington, 30 in Utah)