I am not convinced that the "dark matter" is a sign of any problems. My
master cylinders are both essentially new (rebuilt an in operation < 1
year). Nevertheless, both showed an accumulation of "dark matter" at the
base of the fluid bowls. In my case I am using Dot-4 fluid.
I suspect that this "dark matter" is simply the remnants of normal wear:
e.g. friction of rubber seal on metal causes some displacement of rubber.
I agree it is good practice to bleed the brakes once per season. This gets
any accumulated air and moisture out of the system and also flushes out
"dark matter".
Cheers,
Wayne
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