Leaky self-bailer

My boat came to me with an ELVSTROM/ANDERSEN mini self-bailer which leaked water INTO the boat at all speeds rather than letting it out! It was very useful as an on-land self-bailer as I have yet to manage to completely rain-proof our Mirror. However, the leaking while sailing was such a nuisance that I removed it and temporarily sealed the hole. I checked the mechanism; the little metal flap moved freely and the neoprene seal appeared intact and not overly compressed. The fitting was well-sealed and did not leak (when closed the boat would gradually fill up with rain water).

Does anyone have any advice please on
a) how to prevent backflow of water into the boat through one of these devices (new neoprene seal, vaseline?)
b) how to prevent rain water flowing down shrouds and forestay and past the otherwise sound boat cover?

Any advice gratefully received.

Sail# 68430