Whale Gusher 10.
Sven and all, I have taken the Gusher 10 apart too, several of them and they as well are pretty simple. The guts are two flapper valves and a bug diaphragm essentially. I've found that the consistent problem with them in a marine environment is that salts and aluminum oxide build up beneath the flapper valve pivot points and prohibit them from closing all the way, result: pump no bueno. The fix is simple, it consists of removing the two screws for each flapper, removing the crud and slime and reassembly. The only reason for a rebuild (read expensive as I recall) is if the rubber flappers have bent permanently from the crud or if the big bellows is cracked and leaking air. Hope this helps, Glyn