What is the key aspect of the air brake system to check?

The key aspect of the air brake system to check is the air pressure gauge and the functionality of the brakes. This is crucial because the air pressure gauge provides real-time information about the system's pressure levels, ensuring that the air brakes can function effectively. Air brakes rely on compressed air to operate, and insufficient pressure can result in brake failure or reduced braking power, which poses a significant safety risk.

Monitoring the functionality of the brakes is equally important, as it includes checking for any abnormalities in operation, such as unusual sounds, responsiveness, and application. Ensuring both the gauge is functioning correctly and that the brakes respond as intended confirms that the vehicle can safely stop and operate within safety standards.

Other options relate to different types of braking systems or components that may not be directly relevant to the air brake system. For instance, checking the color of brake fluid is vital for hydraulic systems, but air brakes operate based on air pressure, not fluid. Friction material condition and brake pedal height are relevant checks but are generally more applicable to different braking systems rather than being the focal point within the context of an air brake inspection.