Brake pedal goes to floor with engine running

I would jack up the rear wheels, and adjust the brakes till they jammed the wheels tight,,
if the pedal was no better then,, I'd have a friend pump the brakes twice and hold,, I'd then take the M/C cap off, and have the pedal released,.. which-ever system (fr or rear)shoots fluid back,, I'd bleed that system.. jmo..