Check Valve Replacement. So there is a spring in this valve that only lets water flow one way and prevents water from moving in the other direction when no water flow occurs. Eventually this valve wears out and needs to be replaced. Usually when your irrigation system turns on but you have no water at the sprinkler heads 80% of the time this valve is the problem with well water systems. The pump is loosing its prime. You can unscrew the valve and screw a new one on but I like to cut everything apart and rebuild it all just in case there are pin hole leaks elsewhere and you should trust your own builds more so then anyone else. You can see the nastiness inside the old Valve. The nastiness build up will occur again, but a long time from now. Easy right?