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One method of flushing old fluid that I have read about is every one or two years, bleed the brakes and add about 1/3 the capacity of the master cylinder with fresh fluid 3 or 4. Then, in theory, every 3 to 6 years, you've replaced all your fluid without a major flush. I don't know enough about how the fluid circulates in a closed loop to know if you only need to open up the furthest away bleeder and would get fresh fluid in the entire system.