Automobile & Light Truck Certification Test A5 BrakesDrum Brake Diagnosis and RepairHard
A technician is inspecting a rear drum brake assembly. They notice that the brake shoes are glazed and hardened, and the drum shows signs of minor heat spots and discoloration. The customer reports a 'grabby' or 'hard' brake pedal. What is the MOST likely cause of these symptoms?
- AA faulty master cylinder.
- BImproperly adjusted automatic adjusters.
- CAir trapped in the hydraulic system.
- DOverheating due to excessive braking or improper lining material.
Show answer & explanationAnswer & explanation
Correct answer: D. Overheating due to excessive braking or improper lining material.
Glazed and hardened brake shoes, along with heat spots and discoloration on the drum, are classic signs of overheating. Overheating can be caused by excessive braking, incorrect lining material for the application (too hard), or dragging brakes. Glazing reduces the shoe's friction coefficient, leading to a 'grabby' feel as the material heats and cools inconsistently, and a 'hard' pedal due to reduced initial bite.
Why the other options are wrong
- A. A faulty master cylinder would typically cause a spongy or sinking pedal, not a 'grabby' or 'hard' pedal with these specific shoe/drum symptoms.
- B. Improperly adjusted adjusters would cause a low pedal and excessive travel, not a 'grabby' feel or signs of overheating.
- C. Air in the system would lead to a spongy pedal, not a 'hard' or 'grabby' pedal with signs of overheating.
Brake Overheating Symptoms
Indicators of excessive heat buildup in a braking system, including visual changes to components and changes in pedal feel and braking performance.
- Visual: glazed shoes, discolored/heat-spotted drums/rotors.
- Feel: 'grabby' or 'hard' pedal, brake fade.
- Causes: excessive braking, dragging brakes, incorrect material.
Memory trick: Hot brakes make shoes hard and glazed, and the pedal will feel amazed (or bad!).