Cla-Val Pacific Models 210-09 and 610-09 Combination Altitude and Back Pressure Valves control the high water level in reservoirs without the need for floats or other devices. These valvesmodulate to maintain upstream pressure within close limits to prevent over drawing system supply while filling reservoir. When the shut-off point of the hydraulic pilot control is reached, the valves close smoothly without surges. These valves are designed for one-way flow only.