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CDL · Inspection

How should a driver test that the trailer brakes work on a doubles or triples combination before 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

Charge the air system fully, then check for normal air pressure build-up and proper low-pressure warning, and test the tractor and trailer brakes — including a tug test — to confirm the trailers are connected and braking

  1. A Brakes never need testing on trailers
  2. B Only test the tractor's brakes
  3. C Rely on the previous driver's check
  4. D Charge the air system fully, then check for normal air pressure build-up and proper low-pressure warning, and test the tractor and trailer brakes — including a tug test — to confirm the trailers are connected and braking

Why this is the answer

Before driving, the driver builds the air system to normal pressure and verifies the system works: the low-air warning operates, pressure builds within the proper time, and the brakes apply and release correctly. For the trailers, the driver confirms the air lines are connected and the shut-off valves are set so air reaches all trailer brakes, then performs brake tests — including a gentle tug test at low speed — to confirm the trailers are securely coupled and that their brakes hold. Because a doubles or triples set has multiple trailers and dollies, a missed brake connection on a rear trailer could leave part of the combination without brakes.
Source: FMCSA CDL Manual 7.3.2 Trailer Brake Testing

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About this topic: Inspection

Air brake inspection is its own structured procedure: build-up, leakage rate, low-air warning, and spring-brake activation must all check out before you drive. This is one of 15 Doubles and Triples questions on inspection, 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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