CDL Doubles and Triples Practice Test

25 practice questions

Choose an answer, then check it under Show answer.

1. Question 1 of 25Medium

   After coupling twin trailers and charging the brake system, why must a driver open the emergency line shut-off valve at the rear of the second trailer?

   1. AA. To release the spring brakes on the lead tractor before starting the trip.
   2. BB. To equalize pressure between the lead trailer and the dolly reservoirs.
   3. CC. To confirm air pressure is reaching the back so the brakes will function.
   4. DD. To clear dirt, grease, and excess moisture out of the trailer supply line.
   Show answer to question 1

   Answer: C. To confirm air pressure is reaching the back so the brakes will function.

   Checking for air at the rear emergency shut-off valve verifies that air pressure has reached the back of the entire combination, because without it, the trailer brakes will not work. While petcocks drain moisture from tanks, checking the rear shut-off valve is performed to confirm air supply throughout the train.

2. Question 2 of 25Easy

   When checking that air flows to all trailers, what should you wait for before pushing in the red trailer air supply knob?

   1. AA. Air pressure to reach normal
   2. BB. The trailer handbrake to be released
   3. CC. The service line valve to be opened
   4. DD. The engine to be shut off
   Show answer to question 2

   Answer: A. Air pressure to reach normal

   The procedure says to hold the vehicle, wait for air pressure to reach normal, and then push in the red trailer air supply knob. The handbrake is used later to supply air to the service line, and the engine is not shut off for this check. Opening the service line valve happens at the rear of the last trailer after the emergency line check.

3. Question 3 of 25Medium

   What hazard must you specifically check for before initiating a lane change while pulling double or triple trailers?

   1. AA. Air pressure variation between the front and rear trailer tires
   2. BB. Insufficient air pressure in the secondary supply tank
   3. CC. Lack of adequate clearance along the sides of the vehicle
   4. DD. Excessive operating temperature in the converter dolly wheel hubs
   Show answer to question 3

   Answer: C. Lack of adequate clearance along the sides of the vehicle

   The CDL manual warns that doubles and triples take up more space and cannot be maneuvered suddenly, requiring drivers to be certain they are clear at the sides before changing lanes. Although monitoring air pressure is a standard pre-trip check, verifying side clearance is the critical step before moving across traffic lanes.

4. Question 4 of 25Medium

   What must you confirm regarding the light cords during a walk-around inspection of double or triple trailers?

   1. AA. They are spliced together without plugs
   2. BB. They are coiled loosely over the drawbar
   3. CC. They are firmly in sockets on trailers
   4. DD. They are taped directly to the pintle hook
   Show answer to question 4

   Answer: C. They are firmly in sockets on trailers

   Light cords must be firmly seated in their sockets on the trailers to ensure continuous electrical supply to all trailer lights. Leaving them loosely coiled or unsecured risks disconnecting trailer lighting during travel.

5. Question 5 of 25Medium

   In slippery conditions, what added hazard does a double or triple combination have?

   1. AA. A shorter stopping distance than a single trailer
   2. BB. A greater chance of skids and loss of traction
   3. CC. Less trailer sway at highway speeds
   4. DD. Faster response from the trailer brakes
   Show answer to question 5

   Answer: B. A greater chance of skids and loss of traction

   The manual says doubles and triples have more dead axles to pull with the drive axles, so in bad weather and slippery conditions there is more chance for skids and loss of traction. A shorter stopping distance is not something these combinations gain on slippery pavement.

6. Question 6 of 25Medium

   After coupling twin trailers and charging the brake system, how should you check that air is reaching the second trailer?

   1. AA. Release the parking brake knob on the tractor dashboard.
   2. BB. Disconnect the emergency glad hand between the two trailers.
   3. CC. Open the emergency line shut-off valve at the rear of the second trailer.
   4. DD. Open the air tank petcock on the converter dolly to listen for air.
   Show answer to question 6

   Answer: C. Open the emergency line shut-off valve at the rear of the second trailer.

   The CDL manual instructs drivers to check for air at the rear of the second trailer by opening the emergency line shut-off valve. Opening the petcock on the dolly air tank only checks the dolly reservoir and does not confirm that air is circulating through the second trailer.

7. Question 7 of 25Hard

   During the tractor protection valve test, you build up normal air pressure and push in the air supply knob. What should you do next?

   1. AA. Shut the engine off
   2. BB. Release the tractor parking brake
   3. CC. Pull out the trailer air supply control
   4. DD. Open the emergency line shut-off valve at the rear of the last trailer
   Show answer to question 7

   Answer: A. Shut the engine off

   The manual says to charge the trailer air brake system, then shut the engine off before stepping on and off the brake pedal to reduce air pressure. Pulling out the trailer air supply control would defeat the test, because you are watching for that control to pop out on its own as pressure drops into the manufacturer's range.

8. Question 8 of 25Easy

   During an inspection of double or triple trailers, what must you verify about the air lines and glad hands?

   1. AA. Air lines are spliced and glad hands are left open
   2. BB. Air lines are supported and glad hands are properly connected
   3. CC. Air lines are tied off and glad hands are plugged shut
   4. DD. Air lines are dragging and glad hands are loosely seated
   Show answer to question 8

   Answer: B. Air lines are supported and glad hands are properly connected

   You must be sure that air lines are supported so they do not drag and that glad hands are properly connected between the vehicles. Letting lines drag or leaving connections loose can lead to severed air lines and air brake failure.

9. Question 9 of 25Easy

   How should a driver release the brakes on a converter dolly that is not equipped with spring brakes in order to move it by hand?

   1. AA. Pull the secondary fifth wheel release arm outward.
   2. BB. Open the air tank petcock on the dolly.
   3. CC. Disconnect the dummy gladhand coupler from the tongue.
   4. DD. Back off the manual slack adjusters on the dolly axle.
   Show answer to question 9

   Answer: B. Open the air tank petcock on the dolly.

   To move a converter dolly by hand that lacks spring brakes, the driver must release the dolly brakes by opening the air tank petcock. Manually loosening the slack adjusters or pulling the fifth wheel release handle does not exhaust air pressure from the dolly brake chambers.

10. Question 10 of 25Medium

    Why must a driver steer gently and reduce speed when operating doubles or triples on curves and off-ramps?

    1. AA. To ensure the drive axle differential remains fully unlocked
    2. BB. To keep the converter dolly warning lamp illuminated
    3. CC. To prevent air pressure loss in the trailer emergency lines
    4. DD. To prevent the trailers from rolling over
    Show answer to question 10

    Answer: D. To prevent the trailers from rolling over

    The CDL manual states that to prevent trailers from rolling over, drivers must steer gently and go slowly around corners, ramps, and curves. Speeds that are safe for a straight truck or single trailer combination can easily cause doubles or triples to roll over.

11. Question 11 of 25Easy

    What should you check about the converter dolly air tank drain valve?

    1. AA. It should be removed
    2. BB. It should be closed
    3. CC. It should be halfway open
    4. DD. It should be open
    Show answer to question 11

    Answer: B. It should be closed

    The CDL manual lists the converter dolly air tank drain valve as a check and says it should be closed. Leaving it open would not match the required inspection condition.

12. Question 12 of 25Medium

    In normal operation, what controls the trailer service brakes?

    1. AA. The trailer air supply knob
    2. BB. The foot pedal
    3. CC. The hand control
    4. DD. The emergency line shut-off valve
    Show answer to question 12

    Answer: B. The foot pedal

    The CDL manual says trailer brakes should be tested with the hand valve but are controlled in normal operation with the foot pedal, which applies air to the service brakes at all wheels. The hand control, or trolley valve, is for testing, not for routine braking.

13. Question 13 of 25Medium

    Besides bad weather and slippery roads, in which driving condition must an operator of double or triple bottoms be especially careful to avoid skidding and losing traction?

    1. AA. Expressway driving
    2. BB. High-speed passing
    3. CC. Rural two-lane travel
    4. DD. Mountain driving
    Show answer to question 13

    Answer: D. Mountain driving

    The CDL manual specifically highlights bad weather, slippery conditions, and mountain driving as adverse situations requiring extreme caution. In mountainous terrain, pulling greater length and more dead axles with the drive axles significantly increases the danger of spinning out or losing traction.

14. Question 14 of 25Medium

    After making a visual check of the dolly coupling, what should the driver do next?

    1. AA. Charge the trailer brakes and check for air at the rear.
    2. BB. Connect the safety chains, air hoses, and light cords.
    3. CC. Raise the landing gear completely off the ground.
    4. DD. Open the shut-off valves at the rear of the first trailer.
    Show answer to question 14

    Answer: B. Connect the safety chains, air hoses, and light cords.

    Once the visual check of the coupling is complete, the driver connects the safety chains, air hoses, and light cords. Raising the landing gear and charging the brakes are later steps in the coupling procedure.

15. Question 15 of 25Hard

    In addition to the walk-around inspection on double and triple trailers, which system inspection must also be performed?

    1. AA. Cargo temperature monitoring systems
    2. BB. Tire pressure monitoring calibration
    3. CC. Inspecting air brake systems
    4. DD. Auxiliary transmission systems
    Show answer to question 15

    Answer: C. Inspecting air brake systems

    Walk-around checks on double and triple trailers must be completed in addition to the standard procedures for inspecting air brake systems. Checking cargo temperature or auxiliary transmissions is not the additional system inspection required by the double and triple inspection guidelines.

16. Question 16 of 25Hard

    During the tractor protection valve test, why do you step on and off the brake pedal several times after shutting off the engine?

    1. AA. To reduce the air pressure in the tanks.
    2. BB. To release the tractor parking brake.
    3. CC. To check that the trailer service brakes apply.
    4. DD. To charge the trailer emergency lines.
    Show answer to question 16

    Answer: A. To reduce the air pressure in the tanks.

    The manual says stepping on and off the pedal reduces air pressure so the trailer air supply control should pop out if the tractor protection valve works. This step is not for applying trailer service brakes or charging the emergency lines.

17. Question 17 of 25Easy

    Why must a driver never unlock the pintle hook while the converter dolly is still positioned under the rear trailer?

    1. AA. The dolly emergency spring brakes will permanently lock and prevent towing.
    2. BB. The second trailer will roll backward and collapse the landing gear support legs.
    3. CC. The trailer air lines will immediately rupture and blow out the gladhand seals.
    4. DD. The dolly tow bar may fly up, causing injury and making re-coupling difficult.
    Show answer to question 17

    Answer: D. The dolly tow bar may fly up, causing injury and making re-coupling difficult.

    Unlatching the pintle hook with the dolly under the rear semitrailer releases trapped forces that can cause the tow bar to fly upward violently, risking personal injury and making re-coupling hard. It does not cause gladhand seals to rupture or permanently lock the dolly spring brakes.

18. Question 18 of 25Medium

    Why do doubles and triples have a greater tendency to roll over than single-trailer combinations?

    1. AA. Because their suspension systems are significantly stiffer
    2. BB. Because their brakes apply unevenly across axles
    3. CC. Because their converter dollies carry less cargo weight
    4. DD. Because of the crack-the-whip effect
    Show answer to question 18

    Answer: D. Because of the crack-the-whip effect

    The CDL manual explains that doubles and triples are more likely to turn over than other combination vehicles because of the crack-the-whip effect. Although mechanical balance and suspension design influence vehicle handling, rearward amplification of steering motion is the underlying cause of this rollover hazard.

19. Question 19 of 25Easy

    When inspecting the shut-off valves in the service and emergency lines at the rear of the last trailer, what position should they be in?

    1. AA. Disconnected
    2. BB. Half open
    3. CC. Closed
    4. DD. Open
    Show answer to question 19

    Answer: C. Closed

    The CDL manual states that during an inspection of double and triple trailers, the shut-off valves at the rear of the last trailer must be closed. Valves at the rear of front trailers are kept open to allow air to reach trailing units, but the final valves must be closed to seal the air system.

20. Question 20 of 25Medium

    While checking air flow at the rear of the last trailer, a driver opens the emergency line shut-off valve and hears air escaping. What does this escaping air indicate?

    1. AA. The entire trailer air brake system is charged with air.
    2. BB. The tractor protection valve has popped out automatically.
    3. CC. The service lines are crossed with the trailer emergency lines.
    4. DD. The emergency lines have developed an unexpected pressure leak.
    Show answer to question 20

    Answer: A. The entire trailer air brake system is charged with air.

    Hearing air escape when opening the emergency line shut-off valve at the rear of the last trailer confirms that air has reached the back and the entire system is charged. Hearing air escape during this test step is normal and expected rather than an indication of an air leak.

21. Question 21 of 25Easy

    Why should the more heavily loaded semitrailer be placed in the first position directly behind the tractor in a twin-trailer combination?

    1. AA. To reduce the air brake application lag time between the trailers.
    2. BB. To allow the dolly fifth wheel locking jaws to close automatically.
    3. CC. To prevent the converter dolly tongue from scraping along the ground.
    4. DD. To ensure the safest handling and stability of the rig on the road.
    Show answer to question 21

    Answer: D. To ensure the safest handling and stability of the rig on the road.

    The CDL manual states that for the safest handling on the road, the more heavily loaded trailer should be in the first position behind the tractor. Positioning heavy cargo at the rear creates dangerous handling issues, but it does not affect dolly ground clearance or cause the fifth wheel jaws to close on their own.

22. Question 22 of 25Hard

    What does a yellow lamp on the left side of a converter dolly indicate?

    1. AA. The dolly is equipped with antilock brakes
    2. BB. The dolly has low air pressure in its tank
    3. CC. The dolly parking brake is still applied
    4. DD. The dolly is approved for triple trailer use
    Show answer to question 22

    Answer: A. The dolly is equipped with antilock brakes

    Converter dollies built on or after March 1, 1998 must have antilock brakes, and these dollies have a yellow lamp on the left side. A low air pressure warning is a different signal.

23. Question 23 of 25Hard

    During the tractor protection valve test, what shows that the valve is working properly?

    1. AA. The trailer air supply control pops out when pressure drops into the manufacturer's range
    2. BB. The trailer rolls freely with the air supply knob pushed in
    3. CC. The low air pressure warning stays silent as pressure drops
    4. DD. Air escapes from the emergency line valve at the rear of the last trailer
    Show answer to question 23

    Answer: A. The trailer air supply control pops out when pressure drops into the manufacturer's range

    The manual says the trailer air supply control should pop out, or go from normal to emergency position, when air pressure falls into the range specified by the manufacturer. Air escaping at the rear of the last trailer is part of the air flow check, not the tractor protection valve test.

24. Question 24 of 25Hard

    If a spare tire is carried on the converter gear (dolly), what must you check?

    1. AA. That it is mounted on the last trailer
    2. BB. That it is left loose for quick access
    3. CC. That it is secured
    4. DD. That it is stored inside the cab
    Show answer to question 24

    Answer: C. That it is secured

    The CDL manual says that if a spare tire is carried on the converter gear, you should make sure it is secured. A loose spare tire is not properly secured and could become a hazard.

25. Question 25 of 25Hard

    What requirement applies to converter dollies built on or after March 1, 1998?

    1. AA. They may be equipped with antilock brakes at the owner's option.
    2. BB. They must have a red warning lamp on the left side.
    3. CC. They are required to have a yellow lamp on the right side.
    4. DD. They are required to have antilock brakes.
    Show answer to question 25

    Answer: D. They are required to have antilock brakes.

    The manual says converter dollies built on or after March 1, 1998, are required to have antilock brakes. Such dollies have a yellow lamp on the left side, not a red warning lamp or a yellow lamp on the right side.

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