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Difficulty: MediumBrake System Components Inspection

With the vehicle parked on level ground, wheels securely chocked, and parking brakes released, a driver tests the manual slack adjusters on an S-cam drum brake system by pulling firmly on each pushrod by hand. Which finding indicates that the foundation brake is out of adjustment and requires mechanical service?

  1. A
    The pushrod moves approximately 1/2 inch from the brake chamber.
  2. The pushrod moves more than 1 inch where it attaches to the slack adjuster.Answer
  3. C
    The angle between the pushrod and slack adjuster arm forms approximately 90 degrees when service brakes are applied.
  4. D
    The pushrod moves approximately 3/4 inch when pulled firmly by hand.

Answer

The pushrod moves more than 1 inch where it attaches to the slack adjuster.
When parking brakes are released and the vehicle is securely chocked, manual slack adjusters should not move more than about 1 inch at the point where the pushrod attaches. Movement exceeding 1 inch signifies excessive pushrod stroke, meaning the foundation brake requires adjustment or servicing.

Step-by-Step Solution

1
Identify the standard inspection conditions for checking manual slack adjuster free play.
The vehicle must be on level ground with wheels chocked and parking brakes released so the pushrod is free to move.
Spring brakes must be disengaged to allow manual movement of the adjuster arm.
2
Evaluate the regulatory and safety free play threshold for manual slack adjusters.
Free play travel should not exceed 1 inch (approximately 2.54 cm) when pulled hard by hand.
Excessive pushrod stroke reduces braking efficiency and can lead to brake failure under heavy application.
3
Compare the evaluated conditions against the inspection options.
A pushrod travel exceeding 1 inch indicates the brake is out of adjustment.
Measurements of 1/2 inch and 3/4 inch are within safe limits, and a 90-degree working angle confirms proper setup.

Key Concept

Manual slack adjuster free play inspection threshold on commercial air brake systems
Estimated Time:1m 0s
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Topics

Brake System Components Inspection

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