Cleaning a plastic brake reservior

[William Hartwell Woodruff <woodruff@engin.umich.edu>]

        Do any of you have a good way to get your plastic
brake reservior clean?  I've got my master cylinder apart.
The stuff I can't get rid of is this black film on the bottom
of the reservior.  Where I can reach it with a toothbrush, it 
comes off very easily.  I've tried soapy water and a few 
household cleaners, but nothing seems to dissolve or suspend 
this crud.  Thanks...

woodruff@caen.engin.umich.edu