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CDL Air Brakes Test

Using air brakes: practice questions

Our CDL Air Brakes bank has 38 questions on this topic. Here are 5 of them with answers; the full mock exam draws from every topic, like the real one.

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Question 1

Why should you be in the proper low gear before starting down a hill?

  1. So the compressor can build more air pressure at higher engine speed
  2. So the ABS can work on all axles
  3. So engine braking does most of the work and the brakes don't overheat
  4. So the spring brakes stay released
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C. So engine braking does most of the work and the brakes don't overheat — Brakes are only a supplement to engine braking on downgrades; relying on them too much overheats them and causes fade (CDL Manual 5.4.5, 5.4.6)

Question 2

Why should you check brake adjustment often?

  1. Automatic adjusters only work when checked
  2. Brakes can get out of adjustment quickly, especially when hot
  3. Adjustment changes the governor settings
  4. Checking warms the brakes before driving
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B. Brakes can get out of adjustment quickly, especially when hot — Brakes can get out of adjustment quickly, especially when they are hot, so check adjustment often (CDL Manual 5.4.5)

Question 3

On a long, steep downgrade, what is the role of the service brakes?

  1. The main way to control speed, with the engine doing very little
  2. Only a supplement to the braking effect of the engine
  3. Only for emergencies, never for speed control
  4. To keep the trailer straight while coasting
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B. Only a supplement to the braking effect of the engine — The use of brakes on a long or steep downgrade is only a supplement to the braking effect of the engine (CDL Manual 5.4.6)

Question 4

Without ABS, what happens when your steering wheels lock up?

  1. The vehicle stops in a shorter distance
  2. The trailer swings out but steering stays
  3. You lose steering control
  4. The spring brakes release
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C. You lose steering control — When your steering wheels lock up, you lose steering control; other wheels locking can cause skids or jackknifes (CDL Manual 5.4.2)

Question 5

Only the trailer has ABS, and you start a tractor jackknife. What should you do?

  1. Brake harder so the trailer ABS can work
  2. Apply the trailer hand valve fully
  3. Let up on the brakes, if safe, until you regain control
  4. Pull the trailer air supply knob
Show the answer

C. Let up on the brakes, if safe, until you regain control — When only the trailer has ABS, if you lose steering control or start a tractor jackknife, let up on the brakes (if safe) until you regain control (CDL Manual 5.4.2)

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