By the way, I can see the paper gasket in Adriatic's picture of the postwar (blue) launcher. If that unit hasn't been taken apart and reassembled, then I guess it's between the retaining ring and the diaphragm.
Another tip: to reassemble, I've always used an appropriately sized deep socket to press the retaining ring back in place.
Plus, it occurs to me that maybe your launcher isn't breathing properly. You could try loosening the ring just a tiny bit.
When they are too loose, the launcher will rise too quickly.
