Automobile & Light Truck Certification Test A5 BrakesDisc Brake Diagnosis and RepairHard
During a routine brake inspection, a technician notes that the brake pad material on one side of a fixed caliper system is significantly thinner than the pad on the opposite side. The vehicle is equipped with a dual-piston fixed caliper. What is the MOST probable cause of this uneven wear?
- AA seized caliper guide pin.
- BA faulty master cylinder check valve.
- CUneven piston retraction on one side of the caliper.
- DA restricted brake hose to that caliper.
Show answer & explanationAnswer & explanation
Correct answer: C. Uneven piston retraction on one side of the caliper.
In a fixed caliper, the caliper body does not move; instead, pistons on both sides push the pads. If one pad is significantly thinner, it indicates that the piston(s) on that side are not retracting properly after braking, causing continuous light contact or drag. A seized guide pin is for floating calipers, and a restricted hose would affect both pistons on that caliper or prevent application, not just one side's retraction.
Why the other options are wrong
- A. Fixed calipers do not use guide pins; this is characteristic of floating calipers.
- B. A faulty master cylinder check valve would affect the entire hydraulic system's residual pressure, not just one side of a single caliper.
- D. A restricted hose would typically prevent pressure from reaching the caliper fully, leading to reduced braking or a general drag on both pads, not uneven wear between the two pads.
Fixed Caliper Wear
Wear patterns in fixed caliper systems, often indicating issues with individual pistons or their seals.
- Caliper body is rigid, does not move.
- Pistons on both sides of the rotor.
- Uneven wear often points to piston issues.
- Guide pins are not used.
Memory trick: Fixed pistons mean balanced decisions.