I have already replaced the clutch master cylinder and symptoms were:
- The clutch pedal would not hold pressure.
- When I held the pedal down for more than a few seconds, it would lost all the pressure, to a point that the pedal wouldn't even retract. I had to pull it up.
- Just before replacing it, all gear changes had to be done very very fast, otherwise all pressure was gone, not being able to change gear.
- Had no fluid leaks in both master or slave cylinder.
If the slave cylinder goes bad, it will definitely leak.
As for the master cylinder, it could go bad and have no external leaks. It can leak internally due to the self-adjusting mechanism of the clutch pedal, thus not being able to hold pressure.
My best bet is the clutch master cylinder.
I bought a Blueprint master cylinder for about 100€.
So far, it's working great, although the last time I checked the clutch fluid, it was quite dark. I think it's time to replace it again.
EDIT:
Oh, BTW, I wrote a guide(somewhere in the forum) on howto replace the master cylinder, if you can't find it, let me know and I will look for you.