Wilkins 3-375 3" Backflow Preventer. Cast Bronze Backflow Preventers reverse flow of used or dirty water from entering back into the water supply. Back siphonage and backpressure can occur from time to time and this Backflow Preventer is designed to avoid any of those things. The PVB has a bronze body, silicone rubber check valve disc, and a plastic check valve seat. The valve is ideal for use with large boilers and other applications like it, and is actuated by hand. A tethered test cock cap is provided to protect against fouling caused by insects, dirt and debris. Backflow Preventer can also provide protection where a potential non-health hazards exists.