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CDL · Inspection

Where must the air line shut-off (cut-out) valves be open or closed on a set of trailers?

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

The shut-off valves must be open on all trailers and dollies except the last one, where the valves at the rear must be closed, so air reaches every trailer's brakes

  1. A All valves open including the last trailer
  2. B Only the first trailer's valves open
  3. C All shut-off valves closed
  4. D The shut-off valves must be open on all trailers and dollies except the last one, where the valves at the rear must be closed, so air reaches every trailer's brakes

Why this is the answer

Air for the trailer brakes is carried through lines connected trailer to trailer. The shut-off (cut-out) valves that feed the next connection must be open on every trailer and converter dolly except at the very rear of the last trailer, where they must be closed. If a middle trailer's rear valves are mistakenly closed, the trailers behind it lose air to their brakes; if the last trailer's rear valves are left open, air leaks out and the system cannot build pressure. During the inspection the driver verifies the valve positions are correct so that every trailer in the combination has working brakes.
Source: FMCSA CDL Manual 7.3.4 Air System for Doubles

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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.

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