Automobile & Light Truck Certification Test A5 BrakesDrum Brake Diagnosis and RepairHard
A technician is performing a drum brake service. After removing the drum, they notice that the primary brake shoe has significantly less friction material remaining than the secondary brake shoe. What is the most probable cause of this uneven wear?
- AA weak secondary shoe return spring
- BContamination of the secondary brake shoe
- CA seized wheel cylinder piston on the primary shoe side
- DIncorrect installation of the automatic adjuster
Show answer & explanationAnswer & explanation
Correct answer: C. A seized wheel cylinder piston on the primary shoe side
The primary shoe is the leading shoe, and a seized wheel cylinder piston on that side would prevent it from retracting fully after braking, causing it to drag and wear down faster than the secondary shoe. The primary shoe typically wears faster anyway due to self-energization, so accelerated wear suggests an issue like dragging.
Why the other options are wrong
- A. A weak secondary shoe return spring would cause the secondary shoe to drag and wear excessively, not the primary shoe.
- B. Contamination on the secondary shoe would reduce its friction, causing the primary shoe to do more work, but not necessarily cause the primary shoe to drag and wear excessively.
- D. Incorrect automatic adjuster installation usually leads to both shoes being out of adjustment or one being over-adjusted, not specifically one shoe wearing much faster due to dragging.
Uneven Drum Shoe Wear
Uneven wear between primary and secondary drum brake shoes can indicate issues with specific components, such as seized wheel cylinder pistons, weak return springs, or contamination affecting one shoe more than the other.
- Primary shoe usually wears faster due to self-energization.
- Excessive wear on primary can indicate dragging.
- Excessive wear on secondary can indicate various issues like weak spring or contamination.
Memory trick: Primary's Piston Stuck, Friction's Luck