i had a very similar problem after driving the clutch pedel would change in pick up point, after thinking the same thing that is was either the master or slave cylinder it didn't fix it, i pulled the gearbox off, the throwout bearing was stuffed and also the clutch faces had worn out, i haven't put it all back together as i ordered a new clutch and throw out bearing, however after talking to a few places they said that a worn throw out bearing will cause this.