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Inspecting doubles and triples and checking their air brakes

Doubles and Triples (T)4 minManual section 7.3–7.4

Inspect a double or triple with the normal seven-step method, plus all the combination items, plus a short list that only multi-trailer rigs have: shut-off valves, the dolly drain, the pintle hook and safety chains. Then run the four combination brake tests.

Coupling system and landing gear (7.3.1)

Do these in addition to the walk-around in the general inspection. Many items are just more of the same (tires, wheels, lights, reflectors). Check each fifth wheel in the set:

  • Lower fifth wheel: securely mounted, no missing/damaged parts, enough grease, no visible space between upper and lower fifth wheel, jaws around the shank, not the head, of the kingpin, release arm seated and safety latch/lock engaged.
  • Upper fifth wheel: glide plate securely mounted; kingpin not damaged.
  • Air and electric lines: cord firmly plugged in and secured; air lines connected to glad hands with no leaks and enough slack for turns; no damage.
  • Sliding fifth wheel: no damage or missing parts, greased, no air leaks if air powered, all locking pins present and locked, not so far forward that the frame hits the landing gear or the cab hits the trailer in turns.
  • Landing gear: fully raised, not bent or damaged, crank handle in place and secured, no air or hydraulic leaks if power operated.

Checks only for doubles and triples

  • Shut-off valves (service and emergency lines, at the back of each trailer): behind any front trailer = OPEN; behind the last trailer = CLOSED. In triples, the middle trailer counts as a "front" trailer, so its valves are open.
  • Converter dolly air tank drain valve: CLOSED.
  • Air lines supported and glad hands properly connected.
  • If a spare tire is carried on the converter gear (dolly), make sure it is secured.
  • The pintle eye of the dolly is in place in the pintle hook of the trailer(s), and the pintle hook is latched.
  • Safety chains are secured to the trailer(s).
  • Light cords are firmly in their sockets on the trailers.

Doubles/triples air brake check (7.4)

Check the brakes the same way as any combination, plus make sure air reaches every trailer.

  • Air flows to all trailers: hold the rig with the tractor parking brake and/or chocks. At normal pressure, push in the red trailer air supply knob (fills the emergency lines) and use the trailer hand brake (puts air in the service line). At the rear of the last trailer, open the emergency line shut-off — you should hear air escape. Close it. Open the service line valve to check service pressure passes through all trailers, then close it. No air from both lines? Check that the shut-off valves on the trailers and dollies are OPEN.
  • Tractor protection valve: charge the system, shut off the engine, and pump the brake pedal several times. The air supply control should pop out (or go to "emergency") in the maker's range, usually 20–45 psi. If it does not, a leak could drain all the tractor's air and set the emergency brakes, with possible loss of control.
  • Trailer emergency brakes: charge the system and check that the trailer rolls freely. Stop, pull out the air supply control (or set "emergency"), and pull gently with the tractor to check that the emergency brakes hold.
  • Trailer service brakes: normal pressure, release the parking brakes, move forward slowly, and apply the trailer brakes with the hand control (trolley valve). You should feel them come on. Test with the hand valve; in normal driving use the foot pedal, which brakes all wheels.

References and scope

The topic reference is listed below. Sources support verification; the lesson itself is here. The current content needs technical and language review before public launch.

Manual reference: 7.3–7.4

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

Educational questions with explanations. The answer review also works without the interactive quiz.

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Review questions with answers

  1. Which inspection procedure do you use for doubles and triples?
    1. A shorter version, since trailers are similar
    2. Only the air brake check
    3. Only the coupling check
    4. The seven-step procedure, plus the extra combination and multi-trailer items
    Show answer
    The seven-step procedure, plus the extra combination and multi-trailer items

    Use the seven-step inspection, plus the added items for combinations and doubles/triples.

  2. In a set of doubles, the shut-off valves at the rear of the FIRST trailer should be:
    1. Closed
    2. Open
    3. Half open
    4. Removed
    Show answer
    Open

    Rear of front trailers: OPEN. Rear of last trailer: CLOSED.

  3. In a set of triples, the shut-off valves at the rear of the MIDDLE trailer should be:
    1. Open
    2. Closed
    3. Open only for the service line
    4. Closed only for the emergency line
    Show answer
    Open

    The middle trailer is a front trailer, so its rear shut-offs are open; only the last trailer's are closed.

  4. What position should the converter dolly air tank drain valve be in when driving?
    1. Open
    2. Half open
    3. Closed
    4. It does not matter
    Show answer
    Closed

    Converter dolly air tank drain valve: CLOSED.

  5. What should you check on the pintle hook during inspection?
    1. That it is greased like a fifth wheel
    2. That it is open
    3. That the ABS lamp is near it
    4. That the dolly pintle eye is in place and the hook is latched
    Show answer
    That the dolly pintle eye is in place and the hook is latched

    Be sure the pintle eye of the dolly is in the pintle hook and the pintle hook is latched.

  6. If a spare tire is carried on the converter dolly, what must you check?
    1. That it is inflated to 120 psi
    2. That it is secured
    3. That it is removed before driving
    4. That it touches the ground
    Show answer
    That it is secured

    If a spare tire is carried on the converter gear, make sure it is secured.

  7. During inspection of doubles, the safety chains should be:
    1. Secured to the trailer(s)
    2. Removed for highway driving
    3. Dragging on the ground
    4. Connected only on triples
    Show answer
    Secured to the trailer(s)

    Safety chains should be secured to the trailer(s).

  8. When inspecting doubles, what should you check about the light cords?
    1. That they are coiled on the dolly
    2. That they are unplugged to save power
    3. That they are firmly in their sockets on the trailers
    4. That they are crossed with the air lines
    Show answer
    That they are firmly in their sockets on the trailers

    Be sure light cords are firmly in the sockets on the trailers.

  9. On each fifth wheel in a set of doubles, the locking jaws must close around the kingpin's:
    1. Head
    2. Base plate
    3. Glide plate
    4. Shank
    Show answer
    Shank

    Locking jaws must be around the shank, not the head, of the kingpin.

  10. When checking that air flows to all trailers of a double, where do you open the shut-off valves?
    1. At the front of the first trailer
    2. At the rear of the last trailer
    3. On the tractor dash
    4. On the dolly tongue
    Show answer
    At the rear of the last trailer

    Go to the rear of the rig and open the shut-off valves at the rear of the last trailer; you should hear air.

  11. During the air flow test, why must the trailer hand brake or brake pedal be applied when you open the service line valve?
    1. To set the ABS
    2. To keep the landing gear up
    3. So there is air in the service line to come out
    4. To close the tractor protection valve
    Show answer
    So there is air in the service line to come out

    The service line only has air when the brakes are applied, so the test assumes the hand brake or pedal is on.

  12. When testing the tractor protection valve on doubles, the trailer air supply control should pop out at about:
    1. 20 to 45 psi
    2. 60 to 80 psi
    3. 100 to 125 psi
    4. Above 150 psi
    Show answer
    20 to 45 psi

    It should pop out in the maker's range, usually 20 to 45 psi.

  13. When testing the trailer service brakes, what tells you they are connected and working?
    1. The ABS lamp goes out
    2. You hear the governor cut out
    3. The air supply knob pops out
    4. You feel the brakes come on when you apply the hand control while moving slowly
    Show answer
    You feel the brakes come on when you apply the hand control while moving slowly

    Move forward slowly and apply the trailer hand control; feeling the brakes come on shows they work.

  14. Before pulling out the air supply control to test the trailer emergency brakes, what should you check?
    1. That the ABS lamp is on
    2. That the trailer rolls freely
    3. That the engine is off
    4. That the shut-off valves are closed
    Show answer
    That the trailer rolls freely

    Charge the system and check that the trailer rolls freely; then pull the control and tug gently.

  15. True or False: You must have air all the way to the back of a double or triple for all the brakes to work.
    1. True
    2. False
    Show answer
    True

    True. If air does not reach the last trailer, its brakes will not work; check that shut-offs are open.

  16. When inspecting a set of doubles, how should the air lines be?
    1. Hanging loose on the ground
    2. Taped together
    3. Supported, with glad hands properly connected
    4. Disconnected at the dolly
    Show answer
    Supported, with glad hands properly connected

    Air lines must be supported, and the glad hands must be connected correctly.