After every race weekend, the brake fluid in the reservoir turns slightly dark (black). It seems to get worse as I let it sit. Could the brake fluid (Wilwood 570) be attacking the seals? There is a small leak at the reservoir seal into the master so this is the suspect.
Car is an '86 2nd gen. Last winter I rebuilt calipers, installed new master cyl. and put in Goodridge SS flex lines. Everything else is original. I bleed before every race weekend. My first thought was water or air contamination, but when I bleed I get no bubbles, and when I race I don't get any noticeable fluid fade.

Thanks,
Mike