What type of contamination would lead to a rejection of a vehicle's brake system?

When assessing the condition of a vehicle's brake system, the presence of oil or grease can significantly compromise its performance and safety. These substances can create a slippery film on brake components, such as pads, rotors, and drums, which can hinder the proper friction needed for effective stopping. A contaminated brake surface due to oil or grease may lead to longer stopping distances, reduced responsiveness, and the potential for brake failure, making it a critical safety issue that warrants rejection during an inspection.

Moisture and dirt can also affect the brake system, but they do not have the same immediate and extreme impact on braking performance as oil or grease. While moisture can lead to rust and corrosion over time, and dirt can cause wear or abrasiveness, neither poses the same level of immediate danger in terms of slippery surfaces. Air is a normal component within hydraulic brake systems and does not constitute contamination in this context, so its presence would not lead to a rejection.