HS41X-A backflow preventer with filter consists of two separated check valves and a hydraulically actuated drain valve. A filter screen is installed inside the first check valve. Due to the local head loss of the check valve, the pressure in the intermediate chamber is always lower than that at the inlet. This pressure difference keeps the drain valve closed during normal water supply. In case of abnormal pressure (i.e., when the outlet pressure exceeds that of the intermediate chamber), even if