Brake pedal goes to floor with engine running

No problem with the cap off, but you have to do it REEEEAL slow; I do it with the caps on for that issue.

To the OP, sometimes the MC can get too full of air if drain and then pumped with no fluid in the reserviors, and you have to bench bleed it again. Not very common in my experience but have had this happen once or twice in 40 years.

One other possibility of the 'pumping up', when the engine is off is that there is residual vacuum in the booster that makes the pedal go down further but you bleed that residual vacuum off with a few pumps. It that is the case, then probably the MC. Sounds like you are on that path now. I bleed the same way, BTW.

Any way the calipers on front are reversed with the bleeders pointing down? Just checkin'....