Well, if you can rule out fluid leaks at this point, here's something to try...

Have someone pump the clutch pedal while you watch the slave & throw-out lever. I don't know if certain years are different, but probably the throw-out lever is supposed to move about 12 or 13 mm back & forth.

Then after pumping, have your helper hold the pedal down on the floor. Carefully measure the position of the T-O lever. If it slowly moves back towards the slave, over maybe 5 or 10 minutes, then your master cylinder is leaking internally back to the reservoir. I think the master is the ONLY place that can leak internally without leaking OUT.

You may have to repeat this test at different times. When this happened to my Integra brake MC, it wouldn't leak down all the time. You may have a hard time catching it in the act of leaking.