[lineslinger]

This probably isn't the answer you want to hear but unless you can see and feel the inside of both the slave and master cylinder it is going to be tough to tell which one, if not both, are causing the problem.
I had the same problem on an austin healy, after putting new seals in both the slave and master clylinder I still had a spongy clutch that kept getting worse.
I ended up sleeving both the slave and master cylinder after finding some very light scoring on the sleeve walls of both cylinders.
Never had a problem again after that.
My point is if I would have rebuilt both of them the first time I would have saved myself a lot of time and "creative language" in the long run.
Good luck