The same thing happened to me when I replaced my slave. Having not bled a clutch before I first tried doing it with the car off, and I could get the fluid without bubbles, but I could never get the stiffness back in the clutch. I did it with the car running and it again bled well, but afterwards the pedal was nice and stiff. I'm sure thats probably what you did, so I'm sure none of this helps. haha.