1990 isuzu trooper - brakes spongy & to floor

x-posted: rec.autos.tech, alt.autos.isuzu - followup set: rec.autos.tech
1990 isuzu trooper (198k miles), v6, 4wd, 4door, 4-wheel disc brakes.
Replaced: all four calipers (REMAN), all new pads, new rotors up front. (no hoses replaced).
Engine off, brakes bled (while vehicle off) using "helper in driver seat" method (started at RR to LR to RF to LF; master cyl not bled).
brake pedal feels nice and tight while engine off and pedal is close to its "top" resting point (when you push on it). start engine and brake pedal moves down about halfway to the floorboard stop, feels spongy, and if you hold down on the pedal, it [eventually] travels towards the floorboard.
is this a "bad" brake bleed? maybe a bad master cylinder? or is the vacuum bad to the power assist? these are guesses on my part ... not sure how to proceed or the order in which to check items.
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