Which of the following is an indication of a malfunctioning air brake system?

Answer: B — There is unusual noise during braking

A malfunctioning air brake system can often lead to unusual noises during braking. These noises can indicate a range of issues, such as air leaks, worn brake components, or imbalanced brake forces. When brakes operate normally, they should function quietly; therefore, unexpected sounds such as hissing, grinding, or squealing can serve as warning signs that something is not working correctly. In contrast, smooth vehicle stopping is a sign of a properly functioning braking system, while brake lights functioning properly indicates that the electrical components related to braking are working, not the air brake system itself. Minimal pressure required for braking might suggest over-assistance or an issue with brake responsiveness, but it does not directly indicate a malfunction as clearly as unusual noises do.