SoCal'beaming
Gold Level Sponsor
A good thorough flush with normal brake fluid should wash the crud out. You may clean the brake master reservoir though with a wiping towel before the refill. Pump out the old fluid into a glass container and reuse it with the old fluid for bleeding. You'll note the difference once fresh fluid comes out with dirt and will finally clear up...
Thanks again for all the help! .... DavidTo install them is a bit fiddly while the added protection value is rather questionable. If you intend to ride in nasty weathers perhaps they make sense. I usually leave them off & never encountered any issue with this - even after heavy thunderstorm rides ...