Which component indicates that the air brake system is functioning properly during tests?

The correct identification of the pressure gauge as an indicator of the air brake system's proper functioning is based on its critical role in measuring and displaying the pressure levels within the brake system. A pressure gauge provides immediate feedback regarding whether there is sufficient air pressure available to operate the brakes effectively. During brake tests, fluctuations in pressure readings can indicate leaks or other issues that may affect braking performance.

While the brake cylinder and reservoir tanks are also important components of the air brake system, they serve different functions. The brake cylinder is responsible for converting air pressure into mechanical force to apply the brakes, and the reservoir tanks are used to store compressed air. However, without the pressure gauge providing real-time readings, it would be challenging to ascertain whether the air brake system is functioning correctly. The pressure gauge specifically focuses on the operational status of the entire system, making it a direct indicator of performance.

Therefore, recognizing the pressure gauge's significance in assessing the air brake system's functionality is crucial for ensuring safety and reliability in railway operations.