Series 757 Double Check Valve Assembly Backflow Preventers are designed to prevent the reverse flow of liquids in a plumbing system due to backpressure or backsiphonage where there is a non-health hazard. Compact design and lightweight stainless steel construction (70% lighter than traditional designs) makes installation easier and faster. For unmatched versatility, assemblies are available for horizontal, vertical, or N pattern installations while groove connections allow ease of installation and pipe alignment. Patented tri-link check assemblies are also designed for easy serviceability. A variety of shutoff valve options are offered.