Automobile & Light Truck Certification Test A5 BrakesDisc Brake Diagnosis and RepairMedium
During a brake inspection, a technician observes fine, hairline cracks radiating from the lug nut holes on a vehicle's brake rotor. The vehicle owner reports no current braking issues. What is the MOST appropriate action for the technician to take?
- AReplace the rotor immediately.
- BBleed the brake system to improve performance.
- CNote the cracks and advise the customer to monitor them.
- DMachine the rotor to remove the cracks.
Show answer & explanationAnswer & explanation
Correct answer: A. Replace the rotor immediately.
Hairline cracks radiating from the lug nut holes, often called 'stress cracks' or 'heat cracks,' indicate structural weakening of the rotor. Even if current performance is unaffected, these cracks can propagate rapidly, leading to rotor failure and catastrophic braking issues. Immediate replacement is necessary for safety.
Why the other options are wrong
- B. Bleeding the brake system will not address a physical defect in the rotor.
- C. Monitoring cracks on a critical component like a brake rotor is unsafe and risks catastrophic failure.
- D. Cracks originating from lug holes are structural and often deep; machining will not remove them entirely and further weakens the rotor.
Rotor Stress Cracks
Hairline cracks, especially those radiating from lug holes or the rotor hat, indicate material fatigue and structural compromise due to excessive heat and stress, necessitating immediate replacement.
- Often caused by rapid heating/cooling cycles.
- Can propagate rapidly, leading to rotor failure.
- Even minor cracks require replacement for safety.
Memory trick: Cracks mean danger, replace with care.