One of the great things about the freezer bag mod is that there is
enough resistance to hold water in a slowly lowered wing, but not enough
to stop the force of any signifigant pressure. I have found that if I
lower the wing too rapidly, I get a "blow out" of the plastic diaphram.
I am now in my third year using this method and it works great. I think
the supposition that this 1mil sheet of plastic is going to create
enough resistance to split the wing is unrealistic.
I made several blow out and reset tests.

Gary Emerson wrote:
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.