Automobile & Light Truck Certification Test A5 BrakesDisc Brake Diagnosis and RepairHard
A technician is preparing to replace the front brake pads on a vehicle. After removing the wheel, they observe that the caliper piston is fully extended and the inner brake pad is completely worn, while the outer pad shows minimal wear. The caliper is a single-piston floating type. What is the MOST likely cause of this condition?
- AThe caliper guide pins are seized.
- BThe master cylinder primary cup is leaking.
- CThe brake hose is restricted internally.
- DThe inner brake pad is binding in the caliper bracket.
Show answer & explanationAnswer & explanation
Correct answer: A. The caliper guide pins are seized.
In a floating caliper, the piston pushes the inner pad, and the caliper body slides on the guide pins to pull the outer pad into the rotor. If the guide pins are seized, the caliper cannot slide. This causes the inner pad to wear rapidly (as the piston extends fully), while the outer pad receives minimal pressure and shows little wear.
Why the other options are wrong
- B. A leaking master cylinder primary cup would cause a spongy pedal or loss of pressure, affecting all brakes, not just one side of a single caliper.
- C. An internally restricted hose would prevent fluid from returning, causing both pads to drag or the piston to not retract, but wouldn't typically cause this specific pattern of uneven wear.
- D. An inner pad binding would cause wear, but if the piston is fully extended and the outer pad is barely worn, it points more strongly to the caliper's inability to move as a whole.
Floating Caliper Uneven Wear
A common diagnostic scenario where one pad wears significantly more than the other in a floating caliper system.
- Often caused by seized guide pins.
- Inner pad wears more if caliper can't pull outer pad.
- Outer pad wears more if caliper can't push inner pad (less common).
Memory trick: Stuck slides mean lopsided rides.