Hi Alan.
I have not seen that problem specifically but have you removed the calipers and pulled the pistons out?
It's amazing what you can find in these things and they are suppose to be spotlessly clean...
Same goes for the master cylinder.
If you haven't done this it might be a good place to start.
Also, what does your manufacture recommend for the fluid?
I'm assuming Dot 4 but it could make a difference.