Automobile & Light Truck Certification Test A5 BrakesDrum Brake Diagnosis and RepairMedium
A technician is performing a drum brake service on a vehicle. After removing the drum, they notice that the friction material on the primary shoe is significantly thinner than the friction material on the secondary shoe. What is the MOST likely reason for this condition?
- AThe wheel cylinder is leaking on the secondary shoe side.
- BThe automatic adjuster mechanism is seized.
- CThe parking brake cable is improperly adjusted.
- DThe primary shoe is designed to wear faster due to its self-energizing effect.
Show answer & explanationAnswer & explanation
Correct answer: D. The primary shoe is designed to wear faster due to its self-energizing effect.
In a duo-servo drum brake system, the primary shoe is positioned to be 'self-energizing' when the brakes are applied during forward motion. This means the rotation of the drum forces the primary shoe more firmly against the drum, causing it to do more work and consequently wear faster than the secondary shoe.
Why the other options are wrong
- A. A leaking wheel cylinder would contaminate the friction material, leading to reduced braking on that side and potentially uneven wear, but not inherently faster wear on the primary shoe due to its position.
- B. A seized automatic adjuster would typically cause uneven wear between both shoes or premature wear on both, but not consistently faster wear on only the primary shoe.
- C. An improperly adjusted parking brake primarily affects the secondary shoe, as it is the one typically actuated by the parking brake lever.
Primary Shoe Wear
The primary shoe in a duo-servo drum brake system typically wears faster than the secondary shoe due to its self-energizing action during forward braking.
- Self-energizing effect
- Located towards the front of the vehicle
- Does more work during forward braking
Memory trick: Primary leads the dance, secondary follows the trance.