That's what I did, I left it soaking for an hour in warm water with some dish soap but still there was some "smudge" left in the little chamber that I could not get rid off. The reservoir was really dirty, the complete inside bottom was black!

The weird thing was that the fluid in the reservoir was dark brown almost black, the fluid that came out of the right rear had brownish tone with almost no smudge, but all the other lines the fluid that came out was yellow and clear. After I reinstalled the reservoir I topped it of with new brake fluid.

Now another question; I used two liters of DOT-4, but when I cleaned the fill cap, it shows on top: "Use only DOT-3", did I use the wrong grade?

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).