What chemical would you soak a master cyl in?

In the small side is part of the rubber gasket for the lid, see the writing in there?

See those spacer looking things in the ports, those are factory on a 1967 mopar. Reuse them.

First take the piston out, unscrew the spacers, take the fittings out of the ports, use your favorite solvent to get it clean, then evaporust to get the rust out. Now rebuild it.

evaporust doesn’t remove goop, and it lasts much longer if you use it on clean metal.