Free Air Brakes CDL practice questions

Sample questions from this test

1. Air brakes are really three braking systems. Which list is correct?
   1. Service, hydraulic and parking
   2. Primary, secondary and trailer
   3. Service, parking and emergency
   4. Foot, hand and engine
   Show answer

   Service, parking and emergency

   An air brake setup is made of the service brake, parking brake and emergency brake systems.

2. Why must air tanks be drained?
   1. To lower the pressure for the governor test
   2. Water and compressor oil collect in them and can cause brake failure
   3. To let the alcohol evaporator refill
   4. To cool the compressor
   Show answer

   Water and compressor oil collect in them and can cause brake failure

   Water and oil collect at the bottom of the tanks. Water can freeze and cause brake failure.

3. Pressing and releasing the brake pedal when it is not needed can:
   1. Raise the air pressure
   2. Adjust the slack adjusters
   3. Let air out faster than the compressor can replace it
   4. Cool the brakes
   Show answer

   Let air out faster than the compressor can replace it

   Each release lets air out. Unneeded pumping can drain air faster than the compressor makes it, and the brakes may not work.

4. A vehicle with a dual air brake system will have:
   1. A supply gauge for each half of the system, or one gauge with two needles
   2. No supply pressure gauge
   3. Only an application pressure gauge
   4. A gauge on the trailer only
   Show answer

   A supply gauge for each half of the system, or one gauge with two needles

   Dual systems have a gauge for each half, or a single gauge with two needles.

5. On large buses, low pressure warning devices commonly signal at:
   1. 55–60 psi
   2. 80–85 psi
   3. 100–110 psi
   4. 20–45 psi
   Show answer

   80–85 psi

   It is common for large buses to have low-pressure warnings at 80–85 psi.

6. In a dual air system, the low air warning light and buzzer should shut off when pressure in both systems rises above a value set by the maker. That value must be:
   1. Less than 20 psi
   2. Exactly 100 psi
   3. Greater than 55 psi
   4. Greater than 150 psi
   Show answer

   Greater than 55 psi

   The shut-off value is set by the manufacturer and must be greater than 55 psi.

7. In newer air brake vehicles, the parking brake control is a:
   1. Round red knob you push in to apply
   2. Yellow, diamond-shaped knob you pull out to apply
   3. Blue lever you push down to apply
   4. Green button on the steering wheel
   Show answer

   Yellow, diamond-shaped knob you pull out to apply

   Newer vehicles use a yellow diamond-shaped push-pull knob: pull out to apply, push in to release.

8. Where is the ABS malfunction lamp on a trailer?
   1. On the instrument panel of the tractor only
   2. On the right rear corner
   3. Inside the trailer near the doors
   4. On the left side, on the front or rear corner
   Show answer

   On the left side, on the front or rear corner

   Trailers have yellow ABS lamps on the left side, on the front or rear corner.

9. 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.

10. 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.

11. Static leakage test (brakes released, engine off, fully charged system): the loss rate for a single vehicle should be less than:
    1. 1 psi in one minute
    2. 2 psi in one minute
    3. 3 psi in one minute
    4. 4 psi in one minute
    Show answer

    2 psi in one minute

    With brakes released, loss should be under 2 psi/min for single vehicles and under 3 psi/min for combinations.

12. In a dual air system with a failed low air warning, losing air pressure would mainly:
    1. Cause no change at all
    2. Lock all wheels immediately
    3. Increase stopping distance, with only limited braking before the spring brakes come on
    4. Turn on the brake lights
    Show answer

    Increase stopping distance, with only limited braking before the spring brakes come on

    In dual systems, stopping distance increases and only limited braking is possible before the spring brakes apply.

13. For the governor-cut-out applied leakage test, the air pressure should be built up to about:
    1. 120–140 psi
    2. 55–60 psi
    3. 85–100 psi
    4. 150–175 psi
    Show answer

    120–140 psi

    Build pressure to governor cut-out (120–140 psi) before the applied leakage test.

14. Controlled braking means:
    1. Applying the brakes fully, releasing when wheels lock, and re-applying
    2. Using only the parking brake
    3. Pumping the brakes quickly
    4. Applying the brakes as hard as you can without locking the wheels, with very small steering movements
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    Applying the brakes as hard as you can without locking the wheels, with very small steering movements

    Controlled braking: as hard as possible without lockup, small steering moves; release if wheels lock or you must steer more.

15. Your truck has ABS on only one axle. Compared with no ABS at all, what is true?
    1. It gives no benefit unless every axle has ABS
    2. It lets you brake as hard as you want in any emergency
    3. It still gives you more control while braking
    4. It makes the regular brakes stop working
    Show answer

    It still gives you more control while braking

    Even partial ABS (tractor only, trailer only, or a single axle) still gives you more control during braking. Keep braking normally.