Did a bit more looking and it looks like something at grainger would work
http://m.grainger.com/mobile/details/?R=1WPE5
http://m.grainger.com/mobile/details/?R=4DJA7
I kind of like the 1/2" ball check and install vertically to allow flow into the tank. This should allow for normal venting and then seal when upside down.
http://m.grainger.com/mobile/details/?R=1WPE5
http://m.grainger.com/mobile/details/?R=4DJA7
I kind of like the 1/2" ball check and install vertically to allow flow into the tank. This should allow for normal venting and then seal when upside down.