[Putershark]

My mistake, I thoght the one about half way down had one built in... I have though changed my search topic on the Internet to "Duckbill Valve" and I am getting some more hits and ideas Here is the first few I have found so far that may help...

http://www.minivalve.com/htm/DV027.htm

http://www.minivalve.com/

https://www.vernay.com/products/duckbill.htm

http://www.daprorubber.com/subpage.asp?cat=3&id=5

Those links should keep you busy digging around for a while