[JayJay]

I use cork gaskets with a thin coating of Hylomar Universal Blue on each side. I also use (as Tom E suggests) two brass rings to allow slippage between them as the ring is tightened. 100% success.

It's also important to get the back (sealing) side of the glass thoroughly clean and smooth. Putting it in with even the slightest amount of residue (like from the last guy that put it together with Permatex ;-) ) is tempting fate.

JayJay