What should not cover the brake pad surface for a vehicle to be considered safe?

For a vehicle to be considered safe, the surface of the brake pads must be free from any substances that could impair their function. Grease or oil is particularly detrimental because it can severely reduce the friction between the brake pad and the brake rotor. This lack of friction can lead to a significant decrease in braking power, ultimately creating a dangerous situation while driving.

In contrast, dust and moisture can be present on brake pads without necessarily impacting their effectiveness to a significant degree. Dust can actually help in the braking process by providing a certain level of abrasion, while moisture typically evaporates quickly during use. Paint may not be desirable, as it can also interfere with the braking system, but it does not have the same immediate and hazardous impact on safety as grease or oil would. Thus, ensuring that brake pads are free from grease or oil is crucial for maintaining vehicle safety.