1996 Cavalier Clutch problem

About a month ago the clutch fluid res leaked out completely somehow. When this happened my clutch lost all pressure. To put a temporary fix on the problem I had to change the fluid in the res at least twice a week because it would get very dark and dirty. After a while of that I replaced the Master cylinder for the clutch. It seemed to be find for about 4-5 days. Now its lost all pressure again, and the fluid looks perfectly normal. I'm completely stumped on what is causing this. Any ideas?