Jones Stephens B05400K 4" ABS Backwater Valve with Extension Kit and Cast Iron LidJones Stephens B05400K Jones Stephens ABS backwater valves prevent sewage backup in drainage systems by blocking backflow with a flapper that is forced into a closed position if sewage starts accumulating. Their stepped construction encourages water flow, and the flapper and cover have an elastomeric gasket to ensure a watertight seal. With a magnetic strip for underground detection and threaded access cover for easy inspection, maintenance is simple.