A previous pool person closed our pool last year using two of these check valves to plug our lower return lines. The up-side is they are easy to blow the lines - just attach it and pump air through it. It shuts off the back-flow by itself.
I'm a little leery of using them without knowing for sure if they'll hold up to the rigors of a Maryland mid-winter cold-snap. They did work last year, but I don't want to assume that means they'll always work.
Anyone ever use these instead of rubber plugs?