During a brake inspection, a technician finds that one of the rear drum brake shoes is severely worn on the top edge, while the bottom edge shows minimal wear. The opposite shoe in the same wheel assembly shows normal, even wear. Which of the following is the MOST likely cause?
- AA seized or improperly installed hold-down spring.
- BAn over-adjusted parking brake cable.
- CContaminated brake fluid in the hydraulic system.
- DA faulty wheel cylinder causing uneven piston extension.
Show answer & explanationAnswer & explanation
Correct answer: A. A seized or improperly installed hold-down spring.
Uneven wear on a single brake shoe, specifically top-to-bottom, strongly suggests that the shoe is not sitting squarely against the backing plate or is not able to move freely. A seized or improperly installed hold-down spring would prevent the shoe from pivoting correctly, causing it to contact the drum at an angle, leading to localized wear on one edge. A faulty wheel cylinder might cause uneven wear between shoes or across the entire shoe if pistons are stuck, but localized top/bottom wear points to the shoe's mounting.
Why the other options are wrong
- B. An over-adjusted parking brake cable would affect both shoes or cause general drag, not specific top-to-bottom wear on one shoe.
- C. Contaminated brake fluid would affect the hydraulic system generally, not cause localized wear on a single shoe.
- D. A faulty wheel cylinder might cause uneven wear between the primary and secondary shoes, or if a piston is seized, the entire shoe might not actuate, but not typically localized top/bottom wear on one shoe.
Drum Brake Shoe Uneven Wear
Non-uniform wear patterns on drum brake shoes indicate specific mechanical issues related to shoe movement, contact, or adjustment.
- Top/bottom wear on one shoe: often hold-down spring or backing plate issue.
- Uneven wear between primary/secondary: adjuster or wheel cylinder problem.
- Conical wear: bent axle flange or drum runout.
Memory trick: Wear on just one edge? It's often the shoe's seating, not the fluid's greeting.