What is the main function of a check valve in an air brake system?

Answer: B — To prevent air flowing back from the service tank to the supply tank

The main function of a check valve in an air brake system is to prevent air from flowing back from the service tank to the supply tank. This is crucial for maintaining pressure within the system and ensuring that the air pressure required for brake operation is preserved. When the driver applies the brakes, air is directed from the supply tank to the service tank, which then activates the brake components. If air were allowed to flow back into the supply tank from the service tank, it could lead to a drop in air pressure and potentially cause brake failure or decreased performance. Thus, the check valve serves as a one-way mechanism, enabling air to travel in only the required direction and ensuring the efficiency and safety of the braking system. The other options involve functions that are not specific to the role of a check valve in an air brake system. For instance, allowing air to flow freely pertains more to other components, while regulating air pressure and increasing braking force relate to different aspects of the system, such as pressure modulation and the function of brake chambers, respectively.