Automobile & Light Truck Certification Test A5 BrakesDrum Brake Diagnosis and RepairHard
A vehicle with drum brakes pulls sharply to one side when the brakes are applied. Upon inspection, the technician finds that the brake drum on the pulling side is significantly hotter than the drum on the opposite side. What is the most likely cause?
- AAn incorrectly adjusted automatic adjuster on the cooler side
- BA plugged proportioning valve on the pulling side
- CA weak return spring on the cooler side
- DA seized wheel cylinder on the pulling side
Show answer & explanationAnswer & explanation
Correct answer: D. A seized wheel cylinder on the pulling side
If the drum on the pulling side is hotter, it indicates that the brake on that side is doing more work, likely dragging or applying more force than intended. A seized wheel cylinder would prevent the shoes from retracting fully, causing constant contact with the drum and generating excessive heat.
Why the other options are wrong
- A. An incorrectly adjusted automatic adjuster on the cooler side might cause less braking on that side, but wouldn't make the pulling side hotter.
- B. A plugged proportioning valve on the pulling side would reduce hydraulic pressure to that brake, causing less braking and less heat.
- C. A weak return spring on the cooler side would cause that brake to drag and get hot, not the pulling side.
Brake Pull Diagnosis
Diagnosing a brake pull involves identifying which brake is over-applying or under-applying, often by checking drum/rotor temperature and inspecting components for issues like seized calipers/wheel cylinders or contaminated friction material.
- Pulling indicates an imbalance in braking force.
- Hotter drum/rotor indicates the side doing more work (dragging or over-applying).
- Cooler drum/rotor indicates the side is under-applying.
Memory trick: Temp Tells the Tale of the Pulling Trail