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Decided this would be a good day to flush old fluid and replace with DOT 4 synthetic. Not my first brake job, by a long shot... but anyway, started with removing some old fluid from MC reservoir then refilling with new. Next, began bleeding brakes at right rear and spent a lot of time there, pumping brakes and draining old fluid, until new clean fluid appeared, and then until all air was removed. Left rear went faster, but... when I started on right front, things just didn't feel right anymore... pedal would go way down and not seeing much fluid pressure at any of the bleeder valves—some, but not much. Never felt like I could get any of the four bled properly after that. (Kept reservoir topped off entire time and never let it get very low.) Been at this a few hours now and don't feel like I'm getting any closer. Not seeing any fluid leaks anywhere. Does this sound like a bad master cylinder possibly? Not sure how old the unit is—looks to be AISIN. Is it possible for an old MC to go bad in middle of job, due to lots of heavy pumping? Appreciate any ideas 🙂