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CDL · Air Brake Parts

What is a slack adjuster in an air brake system, and why does it need to be properly adjusted?

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 component that connects the brake chamber pushrod to the brake cam; if out of adjustment, the brakes may not apply fully, reducing braking effectiveness

  1. A A type of tire
  2. B The component that connects the brake chamber pushrod to the brake cam; if out of adjustment, the brakes may not apply fully, reducing braking effectiveness
  3. C A fuel valve
  4. D A device to adjust the seat

Why this is the answer

The SLACK ADJUSTER connects the brake chamber pushrod to the brake S-CAM (which spreads the brake shoes against the drum). It 'adjusts the slack' to keep the proper clearance between the brake shoes and drum as the linings wear. CDL air brakes.

Why adjustment matters: if the slack adjuster is OUT OF ADJUSTMENT (too much travel/free play), the pushrod travels too far, the brakes don't apply fully, and braking effectiveness is REDUCED — a serious safety hazard (a common cause of brake-related violations and crashes). AUTOMATIC slack adjusters are now standard but still need to be checked (and shouldn't have to be manually adjusted often — if they do, something's wrong). Knowing the slack adjuster's role and the danger of misadjustment (reduced braking) is commonly tested.

Source: FMCSA CDL Manual — Air Brakes, Slack Adjusters

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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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