98 OBW 2.5 5 speed 159,000 miles. I have had this problem since I have owned the car, The clutch fluid turns dark and nasty after a week after being changed. I have replaced the Slave cylinder and hose to fix the "pedal won't come back up problem" and replaced the fluid of course with new fluid from unopened container. I did this job in August 08. The fluid was dark in about a week or two. So I changed the fluid again from another unopened container and this time I used the synthetic fluid from valvoline. Again the fluid went dark after a few weeks. I checked the seal in the cap and it seems fine. Any ideas before I order a new cap and seal for the clutch master cylinder? Thanks all.

Mine stays clean. If its working fine the seals of the cylinder have to be OK. Strange!

My fluid in my brake master cylinder reservoir looks fine too and I have never changed it. It must be a bad seal on the cap letting moisture in. O well, I think I am going to see about fashioning one by hand before I go to the dealer and order a new one.