Spill Resistant Vacuum Breaker Backflow valve The Wilkins Model 460 is designed for installation on indoor and outdoor point-of-use applications to protect against back siphonage of contaminated water into the water supply. During start-up and operation, a separate diaphragm seals the air inlet from the water supply preventing spillage. The assembly is designed for use under constant pressure and provides protection where a potential health hazard exists. These valves much be installed a minimum of 300mm above the highest downstream outlet points.