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CDL · Air Brake Parts

What does the term 'dual air brake system' mean?

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

Two separate air brake systems (typically one for the front, one for the rear) that share one set of controls, providing a backup if one system fails

  1. A Two drivers operate the brakes
  2. B Two separate air brake systems (typically one for the front, one for the rear) that share one set of controls, providing a backup if one system fails
  3. C A system with no reservoirs
  4. D Brakes that work twice as hard

Why this is the answer

A DUAL AIR BRAKE SYSTEM has TWO SEPARATE air brake systems that use a single set of controls. CDL air brakes. Typically: one system operates the rear brakes, the other operates the front brakes; each has its own reservoirs, lines, and components.

Safety/backup: if one system fails (loses air), the OTHER system still provides braking (though with reduced capacity and longer stopping distance) — a redundancy/safety feature. Both systems must be charged (typically to about 100-125 psi) before driving. Modern heavy vehicles use dual air systems for this redundancy. Knowing the dual system provides backup braking if one part fails is commonly tested.

Source: FMCSA CDL Manual — Air Brakes, Dual System

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About this topic: Air Brake Parts

Knowing the parts of the air brake system — compressor, governor, supply tank, dryer, slack adjusters, spring brakes — is required so you can recognize defects during inspection. This is one of 22 Air Brakes questions on air brake parts, 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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