Gunky brake fluid

OK, I didn't need my recently acquired impact driver to remove a broken manifold bolt ... but it did work nicely to remove the frozen and stripped screws from my front brake master cylinder.

The brake was working fine and the fluid level was sufficient. But when I opened the resevoir, I discovered it was full of a brown gunk. Looks vaguely like rust, but not as dark or coarse.

I drained and primed, drained and primed the system a few times until a reasonably clear fluid was ejected at the drain.

Any thoughts as to what the mystery gunk is?
Is there any method (short of replacing the entire assembly and hoses) of cleaning it?

Due to its makeup, brake fluid draws moisture and if not changed frequently will cause metallic parts to rust. Keep changing it annually and you'll probably be fine.

use dot 5, better all around, wont eat paint, dosent attract water and has a higher boiling point