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Air Lines & Electrical Connections — CDL Combination Vehicles Practice Test

Air Lines & Electrical ConnectionsQuestion 1 of 21

21 questions in this topic · 21 drawn at random this round

What color is used for the service air-lines when colors are used to avoid mistakes?

CDL Manual — Combination Vehicles

Questions are based on the official CDL manual (AAMVA model) used by state DMVs. The real CDL knowledge test is given in English — federal rules (FMCSA, 49 CFR 391.11) require English proficiency to drive commercially; our Chinese & Spanish are study aids. Some states also offer the written test in other languages (see the per-state table).

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This CDL Combination Vehicles Air Lines & Electrical Connections practice set has 21 practice questions based on the official exam materials, each with an instant explanation. You need 80% on the real CDL Combination Vehicles knowledge test to pass.

Sample questions

Three questions from the Air Lines & Electrical Connections bank with the correct answer and an explanation. The practice quiz above draws 21 questions at random from the same bank.

  1. What is the trailer hand valve (trolley valve) supposed to be used for?

    • Only to test the trailer brakes — Correct answer
    • As the normal brake while driving
    • To park the combination vehicle
    • To straighten out a jackknifing trailer

    Explanation: The trailer hand valve should be used only to test the trailer brakes. Do not use it in driving because of the danger of making the trailer skid. English term: trailer hand valve（挂车手阀 / válvula de mano del remolque）.

    Handbook: Air Lines & Electrical — Trailer Hand Valve

  2. When you press the foot brake in a combination vehicle, where does the air go?

    • To all of the brakes on the vehicle, including the trailer(s) — Correct answer
    • Only to the tractor brakes, never the trailer
    • Only to the trailer brakes
    • Only to the rear axle of the last trailer

    Explanation: The foot brake sends air to all of the brakes on the vehicle, including the trailer(s). There is much less danger of causing a skid or jackknife when using just the foot brake than when using the hand valve. English term: foot brake（脚刹 / freno de pie）.

    Handbook: Air Lines & Electrical — Trailer Hand Valve

  3. On newer vehicles, what does the trailer air supply control look like?

    • A red eight-sided (octagonal) knob — Correct answer
    • A blue round knob
    • A yellow diamond-shaped knob
    • A green square button

    Explanation: On newer vehicles the trailer air supply control is a red eight-sided (octagonal) knob, which you use to control the tractor protection valve. English term: trailer air supply valve（挂车供气阀，红色八角 / válvula de suministro de aire del remolque）.

    Handbook: Air Lines & Electrical — Trailer Air Supply Control