What indicator shows that an air brake system is not functioning properly?

The best indicator that an air brake system is not functioning properly is the warning light illuminating on the dash. This warning light serves as a crucial alert to the driver that there is a potential issue within the air brake system. It can signify various malfunctioning components such as insufficient air pressure, potential leaks, or other critical failures that could compromise the braking effectiveness. Ignoring this warning can lead to dangerous driving conditions, making this indicator essential for maintaining safety on the road.

In contrast, an air pressure gauge rising indicates that the air brake system is building sufficient pressure, which is a positive sign of functionality. The sound of brakes engaging generally signifies that the system is operating as intended. A brake fluid level indicator is typically more relevant to hydraulic brake systems rather than air brakes, and does not provide feedback specific to the performance of an air brake system. Hence, the warning light is the most critical alert for identifying potential problems in air brakes.