Slave cylinder troubles

[Radek]

Patient:  1988 90Q.
Symptom:  very hard clutch pedal.
Tried to replace master and slave cylinder: however, the new slave would not go in.  No matter how hard I pushed, used a prybar, wiggled the thing, it would not go all the way in.  It felt like it was compressing all it could, no more room. I ended up putting back the old slave, which went in fine, by hand (no need for a prybar).

After reverse bleeding the system (lots of air bubbles coming out for a long time), the clutch is a bit better, but still too hard.  
Any idea what could be causing the brand new slave to be so stubborn?  Could it be the release bearing or something else internal, that will force me to take out the engine?
Thanks.

Radek.