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Inspecting a combination and checking its air brakes

Combination Vehicles4 minManual section 6.5

Use the same seven-step inspection you learned for any truck. A combination simply has more of some things (tires, wheels, lights, reflectors) plus new items: the coupling system, the landing gear, the trailer lines, and four extra brake tests.

Extra walk-around checks (6.5.1)

Do these in addition to the walk-around checks in the general inspection.

Lower fifth wheel: firmly mounted to the frame; nothing missing or damaged; greased enough; no gap you can see between the upper and lower fifth wheel; jaws locked around the kingpin's shank, not its head; release arm seated correctly and safety latch/lock engaged.

Upper fifth wheel: glide plate securely mounted to the trailer frame; kingpin not damaged.

Air and electrical lines to the trailer: electrical cord plugged in tight and secured; air lines hooked to the correct glad hands, not leaking, and secured with enough slack to turn; no damage on any line.

Sliding fifth wheel: slide not damaged, no missing parts; properly greased; if air powered, no air leaks; all locking pins present and locked; fifth wheel not so far forward that the tractor frame will hit the landing gear, or the cab will hit the trailer, in turns.

Landing gear: all the way up; no missing, bent or damaged parts; crank handle stowed and secured; on power-operated gear, no air or hydraulic leaks.

Combination brake checks (6.5.2)

Do these in addition to the normal air brake inspection. Doubles and triples are checked the same way.

1. Check that air flows to all trailers. Hold the rig with the tractor parking brake and/or wheel chocks. Wait for normal pressure, then push in the red trailer air supply knob — this fills the emergency (supply) lines. Use the trailer hand brake to put air in the service line. Walk to the rear. Open the emergency line shut-off valve at the back of the last trailer: you should hear air escaping, which shows the whole system is charged. Close it. Then open the service line valve to check that service pressure reaches through all trailers (the hand brake or brake pedal must be applied), and close it. If you do not hear air from both lines, check that the shut-off valves on the trailers and dollies are OPEN. You must have air all the way to the back or not all brakes will work.

2. Test the tractor protection valve. Charge the trailer air system (normal pressure, knob pushed in). Turn the engine off. Press and release the brake pedal several times to bring tank pressure down. The trailer air supply control should pop out (or move from "normal" to "emergency") once pressure reaches the maker's set range — usually 20 to 45 psi. If this valve fails, a leaking hose or trailer brake could empty all of the tractor's air, and the emergency brakes would come on, possibly with loss of control.

3. Test the trailer emergency brakes. Charge the trailer system and make sure the trailer rolls freely. Next, stop and pull the trailer air supply control out (or set it to "emergency"). Tug gently on the trailer with the tractor to confirm the trailer emergency brakes are holding.

4. Test the trailer service brakes. Confirm normal pressure, release the parking brakes, roll forward slowly, and apply the trailer brakes with the hand control (trolley valve) if there is one. You should feel the brakes take hold — proof that the trailer brakes are connected and working. Remember: test with the hand valve, but in normal driving control the brakes with the foot pedal, which sends air to the service brakes at every wheel.

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: 6.5

Official CDL handbook · Utah ↗StatesOur editorial approach

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

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Review questions with answers

  1. What inspection method should you use on a combination vehicle?
    1. A special 3-step method
    2. Only a brake check
    3. The same seven-step inspection, plus extra combination items
    4. Only a walk-around of the trailer
    Show answer
    The same seven-step inspection, plus extra combination items

    Use the seven-step inspection procedure, adding the items unique to combinations.

  2. During a walk-around, what should you check on the upper fifth wheel?
    1. The ABS lamp
    2. Glide plate securely mounted and kingpin not damaged
    3. Air tank drain
    4. Landing gear crank
    Show answer
    Glide plate securely mounted and kingpin not damaged

    Upper fifth wheel: glide plate securely mounted to the trailer frame, kingpin not damaged.

  3. When inspecting a sliding fifth wheel, what must you make sure of?
    1. That it is as far forward as possible
    2. That it is unlocked so it can move
    3. That the jaws are open
    4. That all locking pins are present and locked in place
    Show answer
    That all locking pins are present and locked in place

    All locking pins must be present and locked; also no damage, greased, and not so far forward the frame hits the landing gear.

  4. Why must a sliding fifth wheel not be too far forward?
    1. The tractor frame could hit the landing gear, or the cab hit the trailer, in turns
    2. The ABS would not work
    3. The trailer would be too low
    4. The air lines would be too long
    Show answer
    The tractor frame could hit the landing gear, or the cab hit the trailer, in turns

    Too far forward, the frame may hit the landing gear or the cab may hit the trailer during turns.

  5. How should the air lines to the trailer be when you inspect them?
    1. Tight, with no slack
    2. Tied to the fifth wheel
    3. Properly connected, no leaks, with enough slack for turns
    4. Crossed for better braking
    Show answer
    Properly connected, no leaks, with enough slack for turns

    Air lines must be properly connected, free of leaks, secured with enough slack for turns, and undamaged.

  6. What should you check on the landing gear during the walk-around?
    1. That it is lowered to the ground
    2. Fully raised, not damaged, crank handle secured, no air or hydraulic leaks
    3. That it touches the tractor frame
    4. That the crank handle is removed
    Show answer
    Fully raised, not damaged, crank handle secured, no air or hydraulic leaks

    Landing gear: fully raised, no missing or bent parts, crank handle in place and secured, no leaks if powered.

  7. How do you test that air flows to all trailers?
    1. Watch the ABS lamp
    2. Pump the brake pedal with the engine off
    3. Pull out the air supply knob and tug
    4. Open the emergency and then service shut-off valves at the rear of the last trailer and listen for air
    Show answer
    Open the emergency and then service shut-off valves at the rear of the last trailer and listen for air

    With air supplied and the hand brake on, open each shut-off at the rear of the last trailer; you should hear air escape.

  8. How do you test the tractor protection valve?
    1. Charge the system, shut off the engine, and step on and off the brake pedal until the air supply knob pops out
    2. Pull gently against the locked trailer brakes
    3. Open the shut-off valve at the rear of the last trailer
    4. Apply the trolley valve while moving slowly
    Show answer
    Charge the system, shut off the engine, and step on and off the brake pedal until the air supply knob pops out

    Lower the pressure by pumping the brakes with the engine off; the knob should pop out, usually at 20–45 psi.

  9. How do you test the trailer emergency brakes?
    1. Pump the brake pedal with the engine off
    2. Check that the trailer rolls freely, then pull out the air supply control and pull gently with the tractor
    3. Apply the trolley valve at highway speed
    4. Open the dolly petcock
    Show answer
    Check that the trailer rolls freely, then pull out the air supply control and pull gently with the tractor

    Charge the system, check it rolls freely, then pull the air supply control and tug gently to confirm the emergency brakes hold.

  10. How do you test the trailer service brakes?
    1. Pull out the air supply knob while parked
    2. Shut off the engine and pump the pedal
    3. Move forward slowly and apply the trailer brakes with the hand control (trolley valve)
    4. Open the service line shut-off valve
    Show answer
    Move forward slowly and apply the trailer brakes with the hand control (trolley valve)

    With normal pressure and parking brakes released, move forward slowly and apply the trailer hand control; you should feel the brakes.

  11. During the air flow check, you hear no air escaping at the rear of the last trailer. What should you check?
    1. The ABS lamp
    2. The fifth wheel grease
    3. The landing gear crank
    4. That the shut-off valves on the trailers and dollies are open
    Show answer
    That the shut-off valves on the trailers and dollies are open

    If no air escapes from both lines, check that shut-off valves on the trailers and dollies are in the OPEN position.

  12. If the tractor protection valve does not work right, what could happen?
    1. The trailer would not couple
    2. A leaking air hose or trailer brake could empty all the air from the tractor
    3. The ABS lamp stays on
    4. The landing gear would drop
    Show answer
    A leaking air hose or trailer brake could empty all the air from the tractor

    A bad tractor protection valve lets a trailer leak drain the tractor's air, setting the emergency brakes with possible loss of control.

  13. In normal driving, which control should you use to apply the trailer brakes?
    1. The foot brake pedal
    2. The trailer hand valve
    3. The trailer air supply knob
    4. The parking brake
    Show answer
    The foot brake pedal

    Test the trailer brakes with the hand valve, but control them in normal driving with the foot pedal, which brakes all wheels.