What function does a check valve serve in a keg?

A check valve in a keg serves the important function of preventing beer from flowing back through the coupler when it is disengaged. This mechanism is crucial for maintaining the integrity of the beer delivery system. When the coupler (the device used to tap the keg) is not connected, it allows the check valve to seal, preventing any beer from leaking out or being siphoned back into the coupler. This helps maintain sanitation and quality by preventing contaminated or stale beer from re-entering the system.

Furthermore, the check valve is essential for maintaining the proper pressure balance within the keg while it is being served. It ensures that once the keg has been tapped, beer can only flow in the intended direction—from the keg to the glass—without any backflow that could compromise the beer's quality.

The other choices involve functions that are not related to the primary purpose of a check valve. For instance, while gas pressure regulation is indeed an important aspect of keg operation, it is typically managed through pressure relief valves or regulators, not check valves. Similarly, allowing beer to flow into the coupler or stopping gas flow are functions handled by other components in the keg system.