Double Ball Water Check Valve
This check valve assembly fits all carbonators. It must be installed in between the brass pump and the stainless carbonator tank to avoid contamination in the water. It’s engraved arrow indicated the direction of water flow. Failure to follow the proper flow of water will result in equipment damage and flooding. It’s double ball design is required by all board of heath departments nationwide. This will have twice the water protection as a single check valve. Most health departments require another vented check valve on the water supply line. You can use this check valve in an in-line application as shown. 
Fully assembled and to install on a carbonator, simply remove the short lower portion.