Automobile & Light Truck Certification Test A5 BrakesDrum Brake Diagnosis and RepairMedium
A vehicle exhibits a 'spongy' brake pedal and requires excessive travel to achieve braking in a drum brake system. The brake shoes are new, and the drums are within specification. Which of the following components is the MOST likely cause of this symptom?
- AContaminated brake fluid in the master cylinder.
- BA leaking wheel cylinder.
- CWorn parking brake cables.
- DA seized automatic brake adjuster.
Show answer & explanationAnswer & explanation
Correct answer: B. A leaking wheel cylinder.
A 'spongy' pedal and excessive travel often indicate air in the hydraulic system or a loss of hydraulic pressure. A leaking wheel cylinder allows brake fluid to escape or air to enter the system, leading to these symptoms. Contaminated fluid in the master cylinder could cause issues, but a leak is more direct for a spongy pedal.
Why the other options are wrong
- A. Contaminated brake fluid could cause problems, but a leak (such as from a wheel cylinder) is a more direct cause of a spongy pedal due to air entry or fluid loss.
- C. Worn parking brake cables primarily affect parking brake function and typically do not cause a spongy service brake pedal.
- D. A seized automatic adjuster would typically cause the shoes to remain too far from the drum, leading to excessive pedal travel, but not necessarily a 'spongy' feel unless it's preventing full actuation.
Wheel Cylinder Failure
A malfunction in the wheel cylinder, often due to internal leaks or piston seizing, leading to compromised hydraulic pressure and braking performance.
- Common causes: worn seals, corrosion, fluid contamination.
- Symptoms: spongy pedal, fluid leaks, uneven braking.
- Requires replacement or rebuild (if applicable) to restore hydraulic integrity.
Memory trick: Spongy pedal? Think fluid, air, or a leak, my dear.