When solving a problem, if you input the same numbers, you will get the same answer. If expecting a a different answer, that is stupidity. You feel you have done a good job of sealing the 'ol korky gasket, 2X, so look elsewhere. Obviously it is leaking closely to that area but is it really coming from the gasket surfaces or the expansion plug under it or above the gasket surface area?
Go powder yer ass-end.---- to be sure
I know it is frustrating BTDT. For me, once it was the expansion plug below, and another time it was the one of the relief valves above that had a compromised "O" ring. Is it possible that the cork gasket area is somehow being kept away from compressing the gasket properly to get a good seal?
To back up:
When did the leak originally start? recently, last year, you purchased it leaking? Just trying to get more input so to add up different numbers to solve the problem.
Is there a crack somewhere in the rear end or the hydro, that normally just don't happen? Think outside of the box. There is a trail of oil leading to the source, that only you can find.