I can't answer your question, but I flushed and replaced my fluid 3 weeks ago and it's already going brown again. The slave cyl gaiter was knackered, so it may be letting dirt get in which has been flung off the front sprocket/chain into that area. I'm going to change the piston and the gaiter just as soon as I get an hour or 2 free.
The clever people will be along in a minute to give you a proper answer
