Belated reply. . .my boat-fixing guy found a check valve w/a pretty soft spring. I think it was originally intended for fuel lines? We caulked it into the port drain right at the cockpit. Works like a charm. The spring is soft enough to let any water than accumulates while sailing to drain out normally. When operating the bilge pump, a tiny amount of water leaks back into the cockpit – totally at the “who cares?” level. Bilge empties quickly and easily, cockpit stays almost completely dry. I don’t recall the name/mfg of the valve, but I’m sure I can find out if anyone wants to know.
Another question – has anyone ever found a way to stop the bilge pump from generating backflow into the cockpit. It seems like the port-side drain pipe should have a flapper or check valve to keep the water flowing out, not in. At least half of what I pump out comes right in the cockpit. Any thoughts on this?