I've been plagued with this issue for years. The black fluid is likely caused by aluminum being rubbed into the fluid - like the black residue that accumulates on a rag while polishing aluminum. It eventually destroys the O rings in the master and slave. Also, if you use DOT5
synthetic fluid, the same thing can happen. Check both to see if fluid is getting around the piston seals. Replace one or both depending on what you find. It will just keep getting worse. It it's not the fluid, you may be overdriving your master by exceeding the max travel.