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CDL · Inspecting Combination Vehicles

What should you do if the trailer's air supply line breaks while driving?

Reviewed by Jordan Morley, Editorial Lead at 247SimpleTests. Written by the 247SimpleTests Editorial Team from the official source material cited on every question.

Correct answer

Get off the road immediately; the trailer's emergency brakes will apply automatically and you must not try to drive on them

  1. A Apply only the trailer brakes
  2. B Continue driving until you can stop safely
  3. C Get off the road immediately; the trailer's emergency brakes will apply automatically and you must not try to drive on them
  4. D Reconnect the line by hand while driving

Why this is the answer

If the trailer air supply line breaks (commonly called a 'glad hand' separation), air pressure in the trailer brake system drops rapidly and the trailer's emergency (spring) brakes apply automatically. This is the fail-safe design: a broken line cannot allow a runaway trailer. The driver should expect a sudden, strong pull and possibly a jackknife if the failure happens during braking. Get off the road as safely as possible — do not try to drive with the trailer emergency brakes applied; this can damage the brakes and may not allow normal driving anyway. After stopping, inspect the air line; commonly a glad hand has popped off the trailer connector, which can be reconnected, but a torn or cut line requires repair before continuing.
Source: FMCSA CDL Manual 6.4 Air Brake Failures

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About this topic: Inspecting Combination Vehicles

The combination-vehicle inspection adds fifth wheel, kingpin, glad hands, and trailer-specific items to the standard pre-trip routine. This is one of 13 Combination Vehicles questions on inspecting combination vehicles, each with a full explanation on a single study page.

New to this exam? Our CDL exam guide explains the format, scoring, and how to prepare.

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