Never seen or heard of 'crystals' in brake system. First thing to do is check owners manual and see if the system required brake fluid (the standard ester based stuff) or hydraulic OIL. Oil will destroy the seals in a system made for the ester stuff, while standard brake fluid will destroy those made for Oil.

The Quatroport used Hydraulic OIL in the brakes, as do Roll Royce, Citroen, and a few others.

If is is the usual 'brake fluid', then isoproply alcohol is safe to use. Not Methanol 'Wood alcohol'. Denatured ethyl alcohol is also OK.

I would disassemble everything and clean it out. If crystals are in the master cylinder -- where else are they and what caused them?