why does my clutch's hydraulic system build up "extra" pressure?

[d r i f t swift]

heres the deal. just finished the 5spd swap, took out the clutch dampener box, rebuilt the slave cylinder, using a used master cylinder from the jy.

now the problem.. bled the clutch thought it was fine until i drove it and realized it was slipping. so i jacked the car up to check if the clutch was disengaging all the way..so i backed off the bleed screw on the slave cylinder and the clutch pushed the slave cylinder rod back in. so i was driving the car with the clutch half way disengaged. the next morning the same thing happened. so my question is why the hell does it do that? it builds up "extra" pressure almost instantly if anyone can help me out id appreciate it thanks.