Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairHard
A technician is troubleshooting a vehicle that exhibits a 'dragging' brake on the front left wheel after the brake pedal is released. The master cylinder is functioning correctly, and the caliper slides are well-lubricated. What is the MOST likely cause of this condition?
- AAn incorrectly adjusted parking brake cable.
- BAn internal leak in the master cylinder.
- CA collapsed flexible brake hose.
- DA restricted residual pressure valve.
Show answer & explanationAnswer & explanation
Correct answer: C. A collapsed flexible brake hose.
A collapsed flexible brake hose acts like a one-way valve. When pressure is applied, it allows fluid to pass to the caliper. However, when the pedal is released, the internal collapse prevents the fluid from returning fully to the master cylinder, trapping pressure in the caliper and causing the brake to drag.
Why the other options are wrong
- A. An incorrectly adjusted parking brake cable affects the rear brakes, not the front unless it's a specific integrated system.
- B. An internal master cylinder leak would cause a sinking pedal, not a dragging brake.
- D. A restricted residual pressure valve would affect the entire circuit or axle, not just one wheel, and is more common in drum brakes.
Collapsed Flexible Brake Hose
A flexible brake hose can internally collapse, acting as a check valve that allows fluid to flow to the caliper but restricts its return, leading to a dragging brake.
- Internal layers of the hose separate or swell.
- Fluid can pass under pressure (pedal applied).
- Return flow is blocked or restricted when pressure drops.
- Causes a dragging brake on the affected wheel.
Memory trick: Dragging brake on one wheel? Check the hose, it might be choking.