If you are trying to disassemble the Master Cylinder and you use brake fluid (or water) to soak the MC, you may be able to reuse the seals in the MC. If you start using other liquids (e.g. penetrating oil or gasoline), you are pretty much accepting that the "rubber" parts in the MC are wasted and will need to be replaced.
You don't want to use a liquid which will cause the seals to swell since this can make it more difficult to remove the piston assembly. The culprit seems to be the dried up brake fluid. You need a liquid which will soften this compound without damaging the seals. I have never found penetrating oil to be effective.
I was just looking at what brake fluid is made of ...... looks like many of them (not the silicone ones) are glycol based. So, it looks like you could soak your MC in a pot of boiling water (or maybe a pot of boiling antifreeze) to get the dried brake fluid to dissolve.
good luck,
Dale