How do I determine if the slave or master cylinder is bad?

I had the same problem with my car, and almost got stranded once. I took out the master and found the spring inside had broke, so it was not drawing back enough. So I had that replaced & all cleaned, and a visual inspection looked all ok inside the master. Then once back in the car, it was even worse (proving that visual inspection does not really work). One thing I did notice when my master was not working was that the break fluid would actually come into the reservoir when pressed. So out came the master again and I finally got a new stock one, and all works great now. Maybe take a look at the plastic reservoir when someone presses the pedal. But don't take the cap off, as mine was shooting out pretty hard (not good for the paint!).

Plus I hope you got all the air out. I used a manual hand pump at the slave which worked geat. Also tighten the slave screw tight and adjust everything correctly.

Recently a Pantera expert (Roland Jackel from Germany) told me to connect the rear brakes air outlet with the air outlet of the clutch slave and then pump the fluid backwards trough the clutch system. This worked for me and i never had a better clutch. Just be carefull and watch that the clutch fluid reservoir not overflows.

It sounds to me as if it would have to be the master cylinder, if the slave was at fault, you would have some evidence of leakage (the only place a slave cylinder can bypass to is to atmosphere). Sounds like the master is leaking internally. Good luck & be careful not to get any fluid on the paint or in your eyes (don't ask how I know about this!).
Paul

Probably the most direct way to test is to disconnect the slave and cap the line. Then step on the pedal. If the pedal gradually goes down, the master is either bypassing internally or is leaking down the firewall & under the carpet and you haven't noticed it.
If the capped master can hold firm foot pressure for a few minutes, its a bad slave. I ALWAYS suggest replacing the slave with a long-throw unit in stainless steel, rather than trying to rebuild a unit that was marginal 30 yrs ago. More travel at the throwout= longer life for your ZF synchronisers.