When I changed to an aftermarket slave cylinder my clutch became very light. I bleed it and that didn't help. I did notice that the actuation rod that came with the aftermarket one was shorter than the stock one (the rod that sticks out of the slave and presses on the relaese fork). I did replace the clutch with an aftermarket at the same time, maybe the pressure plate isn't as strong as the the stock one was?? I have no shifting problems though.