How about for s***s and giggles you take pictures of absolutely everything from the master, to lines- following them their entire path on all feeds and another set of pics of all 4 corners with drums off exposing drums shoes and wheel cylinders, maybe someone will see something amiss...we're...
Failed booster should just become manual brakes and will be hard pedal and excessive force needed to stop. Will have no part in the pumping you have to do to get pedal.
Bench bleeding the master shouldn't take more than a few strokes, it doesn't have the capacity to hold much air. I can't imagine if you're bleeding correctly running that quantity of fluid through and still having air in the system. What I can imagine is the shoes settling in and squaring...