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Inspecting air brakes: belt, slack adjusters, drums and hoses

Air Brakes3 minManual section 5.3, 5.3.1, 5.3.2

You inspect an air brake vehicle with the same seven-step pre-trip method from General Knowledge, plus extra air brake items. Most of them fit into Step 2 (engine compartment), Step 5 (walk-around) and Step 7 (the final air brake check, in the next lesson).

Where air brake checks fit in the seven steps

Use the basic seven-step inspection to inspect your vehicle. A vehicle with air brakes has more things to check than one without them, and each extra item fits into a certain step.

Step 2 – Engine compartment: if a belt drives the air compressor, check that the Air compressor drive belt is in good shape and tight enough.

Step 5 – Walk-around: check the slack adjusters, the brake drums (or discs), linings and hoses.

Step 7 – Final air brake check: do the air brake tests instead of the hydraulic brake check. (See the next lesson.)

Checking slack adjusters on S-cam brakes

  • Park on level ground and chock the wheels so the vehicle cannot move.
  • Release the parking brakes so the slack adjusters are free to move.
  • Wear gloves and pull hard on each slack adjuster you can reach.
  • If a slack adjuster moves more than about 1 inch at the point where the push rod connects, it probably needs adjustment. Adjust it yourself or have it done.

Why it matters: a vehicle with too much brake slack can be very hard to stop. Brakes out of adjustment are the number one problem roadside inspectors find. Be safe — check the slack adjusters.

Automatic slack adjusters

All vehicles built since 1994 have Automatic slack adjusters. They adjust themselves during full brake applications, but they must still be checked.

Automatic adjusters should not need manual adjustment, except when doing maintenance on the brakes and when installing the slack adjusters.

If the push rod stroke on an automatic adjuster goes past the legal brake adjustment limit, it shows a mechanical problem: in the adjuster itself, in the related foundation brake parts, or the adjuster was installed wrong.

Adjusting an automatic adjuster by hand to bring the stroke back within limits usually hides the problem instead of fixing it. Routine manual adjustment can also make most automatic adjusters wear out early. And it is dangerous because it can give you a false sense of security about your brakes.

If brakes with automatic adjusters are out of adjustment, take the vehicle to a repair facility as soon as possible. Manual adjustment should be only a temporary fix in an emergency, because the brake will likely go out of adjustment again soon.

Automatic slack adjusters from different makers do not all work the same way, so read that manufacturer's service manual before troubleshooting a brake adjustment problem.

Brake drums (or discs), linings and hoses

  • Drums or discs must not have any crack longer than half the width of the friction area.
  • Linings (friction material) must not be loose, must not be soaked with oil or grease, and must not be dangerously thin.
  • Mechanical parts must be in place, not broken or missing.
  • Air hoses connected to the brake chambers must not be cut or worn from rubbing.

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: 5.3, 5.3.1, 5.3.2

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

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

Preparing your questions…

Review questions with answers

  1. During Step 2 (engine compartment) on an air brake vehicle, you should also check:
    1. The slack adjusters
    2. The low air warning
    3. The air compressor drive belt, if the compressor is belt-driven
    4. The spring brakes
    Show answer
    The air compressor drive belt, if the compressor is belt-driven

    If the compressor is belt-driven, check the belt's condition and tightness in the engine compartment.

  2. Before checking slack adjusters on S-cam brakes, you should:
    1. Park on level ground, chock the wheels, and release the parking brakes
    2. Set the parking brakes and pump the pedal
    3. Run the engine at fast idle
    4. Drain all air from the tanks
    Show answer
    Park on level ground, chock the wheels, and release the parking brakes

    Chock the wheels on level ground and release the parking brakes so the slack adjusters can move.

  3. When you pull hard on a slack adjuster, it probably needs adjustment if it moves more than about:
    1. 1/4 inch
    2. 1 inch
    3. 2 inches
    4. 3 inches
    Show answer
    1 inch

    More than about one inch of movement where the push rod attaches means it probably needs adjustment.

  4. What is the most common problem found in roadside inspections?
    1. Low tire pressure
    2. Burned-out lights
    3. Missing fire extinguisher
    4. Brakes out of adjustment
    Show answer
    Brakes out of adjustment

    Brakes out of adjustment are the number one problem found at roadside inspections.

  5. Vehicles built since what year have automatic slack adjusters?
    1. 1975
    2. 1990
    3. 1994
    4. 1998
    Show answer
    1994

    All vehicles built since 1994 have automatic slack adjusters.

  6. True or False: Automatic slack adjusters adjust themselves, so they do not need to be checked.
    1. True
    2. False
    Show answer
    False

    False. They adjust during full brake applications, but they must still be checked.

  7. You find a brake with an automatic slack adjuster out of adjustment. What is recommended?
    1. Take the vehicle to a repair facility as soon as possible
    2. Adjust it by hand and consider it fixed
    3. Ignore it until the next annual inspection
    4. Drive with the parking brake partly applied
    Show answer
    Take the vehicle to a repair facility as soon as possible

    An out-of-adjustment automatic adjuster signals a mechanical problem; manual adjustment hides it. Get it repaired soon.

  8. Why is manually adjusting an automatic slack adjuster dangerous?
    1. It makes the brakes lock
    2. It drains the air tanks
    3. It turns off the ABS
    4. It can give a false sense of security while the real problem remains
    Show answer
    It can give a false sense of security while the real problem remains

    Manual adjustment usually masks the mechanical problem, so the driver may wrongly trust the brakes.

  9. When is it acceptable to manually adjust an automatic slack adjuster?
    1. Every morning before a trip
    2. During brake maintenance, installation, or as a temporary fix in an emergency
    3. Whenever the ABS lamp comes on
    4. Never, under any condition
    Show answer
    During brake maintenance, installation, or as a temporary fix in an emergency

    Manual adjustment is for maintenance/installation, or only as a temporary emergency measure.

  10. Brake drums must not have cracks longer than:
    1. 1 inch
    2. 1/4 of the drum width
    3. One half the width of the friction area
    4. The width of the lining
    Show answer
    One half the width of the friction area

    Drums or discs must not have cracks longer than half the width of the friction area.

  11. Which brake lining condition is NOT acceptable?
    1. Soaked with oil or grease
    2. Firmly attached
    3. Of normal thickness
    4. Clean and dry
    Show answer
    Soaked with oil or grease

    Linings must not be loose, soaked with oil or grease, or dangerously thin.

  12. When checking the air hoses connected to the brake chambers, look for:
    1. The color of the hose
    2. The hose brand name
    3. Water inside the hose
    4. Cuts or wear from rubbing
    Show answer
    Cuts or wear from rubbing

    Make sure hoses to the brake chambers are not cut or worn from rubbing.

  13. Why does too much brake slack matter?
    1. It makes the compressor work harder
    2. The vehicle can be very hard to stop
    3. It makes the low air warning come on
    4. It increases fuel use only
    Show answer
    The vehicle can be very hard to stop

    Too much brake slack makes a vehicle very hard to stop.

  14. Why should you NOT routinely adjust automatic slack adjusters by hand?
    1. It is illegal for drivers to touch slack adjusters
    2. It usually hides a mechanical problem and can wear out the adjuster early
    3. It makes the brakes too weak to pass inspection
    4. It turns off the ABS
    Show answer
    It usually hides a mechanical problem and can wear out the adjuster early

    Hand-adjusting an automatic adjuster usually covers up the real fault instead of fixing it, and doing it routinely can wear the adjuster out early. Take the vehicle to a repair shop soon.