Sometimes in fuel, brake and clutch flex lines, carbon extracted from the rubber can stain the fluid after awhile. This is not necessarily an indication of imminent failure, just soaking age. The black plastic reservoir screw-cap may also fail along with its seal. There's a nice aluminum screw cap sold cheap by Pegasus Racing that fits Girling reservoirs. Mine's been on my aluminum Girling clutch master for many decades. This unit has no plastic reservoir but is cast with the body.
I've read on some Forum that certain iron castings are slightly attacked by some brake/clutch fluids. Their fix came from changing fluids. Some OEM Bendix-Italia (Benditalia) cylinders seem to be made of this stuff. All my cylinders are anodized aluminum, all the internals are stainless steel and all flex lines are teflon lined braided stainless. I changed all possible o-rings in the car to Viton or teflon. No leaks but my GM plastic brake reservoir still discolors tannish. I think its the plastic they use.