Automobile & Light Truck Certification Test A5 BrakesDisc Brake Diagnosis and RepairMedium
A customer complains of a noticeable vibration and noise from the rear of their vehicle when the parking brake is applied, but not when the service brakes are used. The vehicle has rear disc brakes with an integrated drum-in-hat parking brake. What is the MOST likely cause?
- AExcessive runout of the rear service brake rotor.
- BContaminated or glazed parking brake shoes.
- CA seized rear caliper piston.
- DA worn rear wheel bearing.
Show answer & explanationAnswer & explanation
Correct answer: B. Contaminated or glazed parking brake shoes.
A vibration and noise specifically when the parking brake is applied, but not the service brakes, strongly points to an issue with the separate drum-in-hat parking brake components. Contaminated (e.g., with oil or grease) or glazed parking brake shoes would cause inconsistent friction, leading to a grabbing sensation and vibration/noise when applied.
Why the other options are wrong
- A. Excessive runout of the service brake rotor would cause pulsation when the service brakes are applied, but not when the independent parking brake is used.
- C. A seized rear caliper piston would cause service brake drag and uneven wear, not specific vibration from the parking brake.
- D. A worn rear wheel bearing would typically cause a constant humming or roaring noise that changes with speed, not specifically tied to parking brake application.
Parking Brake Shoe Contamination/Glazing
The presence of oil, grease, or excessive heat causing a hardened (glazed) surface on parking brake shoes, leading to reduced friction, grabbing, vibration, and noise upon application.
- Reduces friction effectiveness.
- Causes grabbing or vibration.
- Requires cleaning or replacement of shoes.
Memory trick: Parking brake noise, shoes are the cause, check the surface, then applause.