What is a condition that would lead to rejecting a vehicle based on its brake discs or drums?

Substantial cracks in the friction surface of brake discs or drums are a critical issue that can significantly compromise the safety and effectiveness of a vehicle's braking system. Brake components need to maintain structural integrity to ensure reliable performance. Cracks can lead to catastrophic failure, especially under the thermal stress generated during braking. When the friction surface is compromised by cracks, it can cause uneven wear, reduced braking power, and the risk of brake failure while driving, creating a hazardous condition for the driver and others on the road.

In contrast, minor rust might not significantly impact braking performance if it does not penetrate deeply or interfere with the disc's function. Similarly, surface discoloration can be a natural occurrence due to heat and does not necessarily warrant rejection unless it indicates deeper problems. Thin lining, while concerning, might still be within acceptable limits as long as it does not diminish the braking effectiveness below safety standards. Thus, substantial cracks pose the most immediate and serious risk, warranting rejection during inspection.