I tried Bob's back sealing idea. Seems to be holding up well, but it increases the strength needed to open the valve a LOT.
I don't know if its just the picture, but it looks like you're using a pretty strong spring in your tank. The smaller the coil size is, the higher the spring constant. I'd recommend something much weaker with less coils/inch, wire thickness, and a wider diameter if possible. It only has to press enough to seal for the first pump or two, then the rest of the work is done by the air pressure itself.
And why the threaded coupler?