Flushing an old brake system...What's a Safe chemical to use?

I WOULD NOT use any solvent//any kind of solvent will probably destroy the piston seals in the master cylinder /calipers/wheel cylinders or the o rings in your proportioning valve /line lock.... brake fluid itself is a good cleaner//fill the system let it sit a few hours and flush with brake fluid...It will absorb any water in the system..