Brake Fluid Residue Solvent?

[Rizzo Steven D LtCol AETC/A5RU <Steven.Rizzo@RANDOLPH.AF.MIL>]

Good day, Listers!  A friend is having difficulty removing the pistons from
the master cylinders.  There is a very thick sticky substance that is
probably old dried out brake fluid "in there".....  What would be a good way
to get those pistons out?  What solvent would be effective in cleaning up
the old dried out fluid?  My first guess would be to soak them in fresh
fluid for a few days.....   Thank you in advance for your help!

Stever