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Commercial driver’s license (CDL) knowledge tests · California DMV

Using Air Brakes

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Questions and answers

  1. In a normal stop with a manual transmission, when do you push the clutch in?

    Not until the engine rpm is down close to idle

    Keep the clutch out until engine speed is close to idle.

    Section 5 · 5.4.1 – Normal Stops ↗

  2. After a normal stop, what do you do with the transmission?

    Select a starting gear

    When stopped, select a starting gear.

    Section 5 · 5.4.1 – Normal Stops ↗

  3. You brake hard on a slippery surface without ABS and your steering wheels lock up. What do you lose?

    Steering control

    Locked steering wheels cannot steer the vehicle.

    Section 5 · 5.4.2 – Braking With Anti-lock Brakes ↗

  4. Without ABS, what may happen when wheels other than the steering wheels lock up?

    You may skid, jackknife, or even spin the vehicle

    Locked wheels on the other axles can cause a skid, a jackknife, or a spin.

    Section 5 · 5.4.2 – Braking With Anti-lock Brakes ↗

  5. How does ABS help you avoid wheel lock up?

    The computer senses impending lockup and reduces the braking pressure to a safe level

    The ABS computer detects a wheel about to lock and lowers braking pressure so you keep control.

    Section 5 · 5.4.2 – Braking With Anti-lock Brakes ↗

  6. What should you be able to do with ABS, even if you cannot stop faster?

    Steer around an obstacle while braking and avoid skids caused by over braking

    You may or may not stop faster with ABS, but you should be able to steer while braking.

    Section 5 · 5.4.2 – Braking With Anti-lock Brakes ↗

  7. Your rig has ABS on only one axle. What does that give you?

    Still more control over the vehicle during braking

    ABS on only the tractor, only the trailer, or even one axle still improves control. Brake normally.

    Section 5 · 5.4.2 – Braking With Anti-lock Brakes ↗

  8. Only your tractor has ABS. What should you keep your eye on while braking?

    The trailer, and let up on the brakes if it begins to swing out and you can safely do so

    With ABS on the tractor only, you should keep steering control, but the trailer can still swing out.

    Section 5 · 5.4.2 – Braking With Anti-lock Brakes ↗

  9. Only your trailer has ABS. You lose steering control or start a tractor jackknife. What should you do?

    Let up on the brakes, if you can safely do so, until you gain control

    Trailer-only ABS makes the trailer less likely to swing out, but the tractor can still lose control.

    Section 5 · 5.4.2 – Braking With Anti-lock Brakes ↗

  10. How should you brake when you drive a tractor-trailer combination with ABS?

    As you always have

    Brake the same way whether ABS is on the tractor, the trailer, or both.

    Section 5 · 5.4.2 – Braking With Anti-lock Brakes ↗

  11. With ABS, how much braking force should you use?

    Only the force necessary to stop safely and stay in control

    Use only the braking force you need to stop safely and stay in control.

    Section 5 · 5.4.2 – Braking With Anti-lock Brakes ↗

  12. What is the one exception that lets you fully apply the brakes in an emergency stop?

    You always drive a straight truck or combination with working ABS on all axles

    With working ABS on all axles, you can fully apply the brakes in an emergency stop.

    Section 5 · 5.4.2 – Braking With Anti-lock Brakes ↗

  13. Your ABS malfunctions during a trip. What should you do?

    Drive normally, because you still have regular brakes, and get the system serviced soon

    A failed ABS leaves the regular brakes working. Have the system serviced soon.

    Section 5 · 5.4.2 – Braking With Anti-lock Brakes ↗

  14. In an emergency stop, how should you brake?

    In a way that keeps the vehicle in a straight line and allows you to turn if necessary

    Brake so the vehicle stays straight and you can still turn.

    Section 5 · 5.4.3 – Emergency Stops ↗

  15. Which two methods can you use for an emergency stop?

    Controlled braking or stab braking

    The two methods are controlled braking and stab braking.

    Section 5 · 5.4.3 – Emergency Stops ↗

  16. What is controlled braking?

    Applying the brakes as hard as you can without locking the wheels

    In controlled braking you brake as hard as possible while the wheels keep turning.

    Section 5 · 5.4.3 – Emergency Stops ↗

  17. How much should you move the steering wheel during controlled braking?

    Keep steering wheel movements very small

    Steering movements stay very small while braking hard.

    Section 5 · 5.4.3 – Emergency Stops ↗

  18. During controlled braking you need a larger steering adjustment, or the wheels lock. What do you do?

    Release the brakes, then reapply them as soon as you can

    Release the brakes for the larger steering move or the lockup, then brake again as soon as possible.

    Section 5 · 5.4.3 – Emergency Stops ↗

  19. What is the first step of stab braking?

    Apply your brakes all the way

    Stab braking starts with a full brake application.

    Section 5 · 5.4.3 – Emergency Stops ↗

  20. In stab braking, when do you release the brakes?

    When the wheels lock up

    Release when the wheels lock, then apply fully again as soon as the wheels start rolling.

    Section 5 · 5.4.3 – Emergency Stops ↗

  21. After you release the brakes in stab braking, how long can it take for the wheels to start rolling?

    Up to 1 second

    It can take up to 1 second for the wheels to start rolling again.

    Section 5 · 5.4.3 – Emergency Stops ↗

  22. In stab braking, what happens if you reapply the brakes before the wheels start rolling?

    The vehicle will not straighten out

    The wheels must be rolling again before you brake, or the vehicle will not straighten out.

    Section 5 · 5.4.3 – Emergency Stops ↗

  23. What is “brake lag”?

    The time required for the brakes to work after the brake pedal is pushed

    Brake lag is the added delay between pushing the pedal and the brakes working.

    Section 5 · 5.4.4 – Stopping Distance ↗

  24. How long does it take for the air to flow through the lines to the brakes?

    1/2 second or more

    Air brakes need a little time, half a second or more, for the air to reach the brakes.

    Section 5 · 5.4.4 – Stopping Distance ↗

  25. How quickly do hydraulic brakes work compared with air brakes?

    Hydraulic brakes work instantly

    Hydraulic brakes, used on cars and light or medium trucks, work instantly.

    Section 5 · 5.4.4 – Stopping Distance ↗

  26. Which factor is added to total stopping distance for vehicles with air brakes?

    Brake lag distance

    Total stopping distance is perception distance plus reaction distance plus brake lag distance plus braking distance.

    Section 5 · 5.4.4 – Stopping Distance ↗

  27. How many factors make up total stopping distance for vehicles with air brake systems?

    4

    The four are perception, reaction, brake lag, and braking distance.

    Section 5 · 5.4.4 – Stopping Distance ↗

  28. How much does air brake lag add to stopping distance at 55 mph on dry pavement?

    About 32 feet

    At 55 mph on dry pavement, brake lag adds about 32 feet.

    Section 5 · 5.4.4 – Stopping Distance ↗

  29. At 55 mph, for an average driver under good traction and brake conditions, what is the total stopping distance with air brakes?

    Over 450 feet

    With brake lag included, the total at 55 mph is over 450 feet.

    Section 5 · 5.4.4 – Stopping Distance ↗

  30. What causes brakes to fade or fail from excessive heat?

    Using them too much and not relying on the engine braking effect

    Too much use of the service brakes, without the help of engine braking, overheats them.

    Section 5 · 5.4.5 – Brake Fading or Failure ↗

  31. What does excessive heat do to the brakes to cause fade?

    Causes chemical changes in the lining that reduce friction, and expansion of the brake drums

    Heat changes the lining so it grips less, and the drums expand away from the shoes.

    Section 5 · 5.4.5 – Brake Fading or Failure ↗

  32. Some of your brakes are out of adjustment. What happens to the other brakes?

    They can overheat and fade, because they do more than their share of the work

    Brakes out of adjustment stop doing their share, so the rest take the load and can overheat.

    Section 5 · 5.4.5 – Brake Fading or Failure ↗

  33. When can brakes get out of adjustment quickly?

    Especially when they are hot

    Brakes can go out of adjustment quickly, especially when hot, so check adjustment often.

    Section 5 · 5.4.5 – Brake Fading or Failure ↗

  34. What can continued overuse of fading brakes lead to?

    The vehicle cannot be slowed down or stopped

    Brake fade can grow until the vehicle cannot be slowed or stopped.

    Section 5 · 5.4.5 – Brake Fading or Failure ↗

  35. On a long or steep downgrade, what role do the brakes play?

    They are only a supplement to the braking effect of the engine

    The engine in the correct low gear does the main work, and the brakes add to it.

    Section 5 · 5.4.6 – Proper Braking Technique ↗

  36. On a downgrade in the correct low gear, how hard do you apply the brakes?

    Just hard enough to feel a definite slowdown

    Apply the brakes just hard enough to feel the vehicle definitely slow down.

    Section 5 · 5.4.6 – Proper Braking Technique ↗

  37. On a downgrade, when do you release the brakes?

    When speed is about 5 mph below your “safe” speed

    Release the brakes once you are about 5 mph under your safe speed.

    Section 5 · 5.4.6 – Proper Braking Technique ↗

  38. About how long should each brake application last on a downgrade?

    About 3 seconds

    The application that takes you 5 mph below your safe speed should last about 3 seconds.

    Section 5 · 5.4.6 – Proper Braking Technique ↗

  39. Your “safe” speed on a downgrade is 40 mph. How do you brake?

    When speed reaches 40 mph, brake to bring it gradually down to 35 mph, then release

    Do not brake until you reach the safe speed, then reduce speed by about 5 mph and release. Repeat as needed.

    Section 5 · 5.4.6 – Proper Braking Technique ↗

  40. On a downgrade you released the brakes at 5 mph below your safe speed. When do you brake again?

    When your speed has increased to your “safe” speed

    Repeat the application each time speed climbs back to the safe speed.

    Section 5 · 5.4.6 – Proper Braking Technique ↗

  41. The low air pressure warning comes on. What should you do?

    Stop and safely park your vehicle as soon as possible

    There might be an air leak, and controlled braking is possible only while enough air remains.

    Section 5 · 5.4.7 – Low Air Pressure ↗

  42. In what pressure range will the spring brakes come on as air is lost?

    20 to 45 psi

    The spring brakes come on when pressure drops into the 20 to 45 psi range.

    Section 5 · 5.4.7 – Low Air Pressure ↗

  43. Why will a heavily loaded vehicle take a long distance to stop on spring brakes?

    The spring brakes do not work on all axles

    Spring brakes act on only some axles, so a heavy vehicle needs a long distance.

    Section 5 · 5.4.7 – Low Air Pressure ↗

  44. What may happen to a lightly loaded vehicle, or a vehicle on a slippery road, when the spring brakes come on?

    It may skid out of control

    It is much safer to stop while there is enough air to use the foot brakes.

    Section 5 · 5.4.7 – Low Air Pressure ↗

  45. On older vehicles, what may the parking brake control look like?

    A round blue knob or some other shape, including a lever

    Newer vehicles have a yellow diamond-shaped knob. Older ones may have a round blue knob or a lever.

    Section 5 · 5.4.8 – Parking Brakes ↗

  46. You have just come down a steep grade and the brakes are very hot. Should you use the parking brakes?

    No, they can be damaged by the heat

    Let hot brakes cool before using the parking brakes.

    Section 5 · 5.4.8 – Parking Brakes ↗

  47. The brakes are very wet and the temperature is freezing. Why should you not use the parking brakes?

    They can freeze so the vehicle cannot move

    Wet brakes set in freezing temperatures can freeze in place.

    Section 5 · 5.4.8 – Parking Brakes ↗

  48. You cannot use the parking brakes because they are very hot or wet in freezing weather. How do you hold the vehicle?

    Use wheel chocks on a level surface

    Wheel chocks on a level surface hold the vehicle when the parking brakes should not be used.

    Section 5 · 5.4.8 – Parking Brakes ↗

  49. Your brakes are wet. How do you dry them?

    Use the brakes lightly while driving in a low gear to heat and dry them

    Light braking in a low gear heats and dries wet brakes.

    Section 5 · 5.4.8 – Parking Brakes ↗

  50. Your vehicle does not have automatic air tank drains. When do you drain the air tanks?

    At the end of each working day

    Drain the tanks at the end of each working day to remove moisture and oil. Otherwise the brakes could fail.

    Section 5 · 5.4.8 – Parking Brakes ↗

  51. You will be away from your vehicle for only a short time. What must you do before leaving it?

    Apply the parking brakes or chock the wheels

    Never leave the vehicle unattended without the parking brakes applied or the wheels chocked. It might roll away.

    Section 5 · 5.4.8 – Parking Brakes ↗

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