Hopefully thermal expansion will not become an issue....splitting a wing
open because it warmed up in the sun could ruin your day.... Your
"check valve" has the potential to allow such a situation...

Paul M. Cordell wrote:
I am always amazed at how few Schempp-Hirth Glider owners have done this
very simple Mod to their water ballast system. The addition of this 1
way valve to the top cap of the ballast tank plug will prevent water
from flowing out of the tank prior to launch. You can put a wing down
without water flowing out of the vent. The vent will still function
properly during the dumping process.

It’s a simple project using a screw driver, scissors and a plastic
freezer bag. Unscrew the top cap from the rest of the plug mechanism.
Cut a disc out of the freezer bag slightly smaller in diameter than
the cap. Punch a hole in the center of the freezer bag disc of
sufficient size for the screw mechanism to reattach. Reassemble and
you are done.

http://www.gliderforum.com/photos/sh...&currentpos=32

http://www.gliderforum.com/photos/sh...&currentpos=34

http://www.gliderforum.com/photos/sh...&currentpos=35

I have to give credit to Jackie Payne for this mod.