Motorcycle Repair: 1982 Honda CX500C master cylinder bypass port, honda cx500c, approximate diameter

Question
1982 Honda CX500C - I have the master cylinder off and disassembled, the bypass orifice is plugged solid with dried crud.  I have soaked it with Berryman B-12 (carb cleaner) to no avail. Any suggestions for a different solvent (Berryman B-12 contains toluene, methanol, acetone, methyl ethyl ketone)?  Do you know the approximate diameter of the passage? I have read random descriptions of anything from .010 to .015 inches.  I don't want to poke at it with anything larger than the passage, for fear of ruining the casting by making the hole too large.   Thanks for your help.

Answer
Craig, I'd use a copper wire or some other soft wire. That won't damage the aluminum but to be honest, it's not that critical.  As long as you don't attempt to drill it out or something like that.  I've not found any solvent that will take that varnish off.  I ususally end up scraping it out with something.  

Regards
Rich