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Lvl 1

Air Brakes

5 free sample questions of the 55 in this topic. The full set is in the app.

  1. Question 1

    The air compressor's job is to:

    • A.Pump air into the air storage tanks
    • B.Cool the brake drums
    • C.Pressurize the fuel system
    • D.Power the windshield wipers
    Show answer & explanation
    Answer: A. Pump air into the air storage tanks

    The compressor — driven off the engine by gears or a v-belt — keeps the storage tanks (reservoirs) charged.

    Source: California Commercial Driver Handbook (DL 650) — §5.1.1 — Air compressor

  2. Question 2

    The governor stops the compressor from pumping at the "cut-out" pressure of about:

    • A.100 psi
    • B.125 psi
    • C.150 psi
    • D.175 psi
    Show answer & explanation
    Answer: B. 125 psi

    Cut-out is around 125 psi; when tank pressure falls to the cut-in level (around 100 psi), the governor lets the compressor pump again.

    Source: California Commercial Driver Handbook (DL 650) — §5.1.2 — Air compressor governor

  3. Question 3

    The governor "cut-in" pressure — where pumping resumes — is around:

    • A.60 psi
    • B.85 psi
    • C.100 psi
    • D.125 psi
    Show answer & explanation
    Answer: C. 100 psi

    Cut-in ≈ 100 psi, cut-out ≈ 125 psi — the working band the compressor maintains.

    Source: California Commercial Driver Handbook (DL 650) — §5.1.2 — Air compressor governor

  4. Question 4

    Air storage tanks hold enough air to:

    • A.Allow several brake applications even if the compressor stops working
    • B.Run the brakes for one stop only
    • C.Last exactly one hour
    • D.Operate the horn but not the brakes
    Show answer & explanation
    Answer: A. Allow several brake applications even if the compressor stops working

    The reservoirs are the buffer: several brake uses remain available even with a dead compressor.

    Source: California Commercial Driver Handbook (DL 650) — §5.1.3 — Air storage tanks

  5. Question 5

    Why must air tanks be drained?

    • A.Water and compressor oil collect in the bottom — and water can freeze and cause brake failure
    • B.To reset the governor
    • C.To lubricate the drain valve
    • D.Only to pass inspections
    Show answer & explanation
    Answer: A. Water and compressor oil collect in the bottom — and water can freeze and cause brake failure

    Compressed air carries water and oil into the tanks; in cold weather that water can freeze in the system and stop the brakes from working.

    Source: California Commercial Driver Handbook (DL 650) — §5.1.4 — Air tank drains

50 more Air Brakes questions are in the app, along with explanations and flashcards for every one.

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OnRoute is an independent study aid based on the federal CDL standards (49 CFR Part 383), the FMCSA Entry-Level Driver Training theory curriculum (49 CFR Part 380 Appendix A), and related Federal Motor Carrier Safety Regulations. It is not affiliated with, endorsed by, or sponsored by the FMCSA, AAMVA, or any state driver licensing agency. CDL knowledge tests are administered by your state, and state-specific requirements may vary — always consult your state's CDL manual.