I like to flush brake lines with alcohol. Get a gallon of methanol (shellac thinner) at the local hardware store or big box store like Home Depot and pump a quart or two through the system with the master cylinder. Then follow with at least a pint of fresh brake fluid, and bleed the brakes as you normally would. Never, EVER put a petroleum-based solvent into a brake system! The residue will destroy rubber parts. Alcohol and rubber brake parts are pretty compatible, and any leftover alky will evaporate.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!