Crystals in the brake fluid can't be good, to say the least.

I can't think of using anything else other than brake fluid to flush the system. Anything else may contaminate the system and ruin the seals.

For a neglected brake system, it may pay to go futher and disassemble the brake hydraulics at each corner and the master cylinder. Give them a good cleaning, button everything back up, and then give them a good flush with brake fluid. This would make for a good long term fix.

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95 E320 Cabriolet, 169K