I'll try and find the pics. The condom method....The problem can be solved of,, "turning the pump off and preventing the water from flow back out",, by placing a small balloon over the pump out flow opening,,with a very selective size hole in it. The correct size hole will not greatly restrict the water pump out flow but when the pump is off it will collapse in and clog the valve opening and prevent the back flow. get a pack and trial and error with sizing off the hole cut in the tip.

See this thread for pics:
http://www.jaxkayakfishing.com/phpBB/topic16212.html