I'm going to give this a shot. An air bubble is going to give you a 'spongy' pedal, but is not going to cause pedal drop if they work part of the time. Since the rod from the pedal to the MC is essentially a solid link with diaphragms attached to transmit force you can rule out the booster from your search. Since you have no leaks, if the pedal goes to the floor without brake pressure it points to the MC allowing fluid pressure to recirculate back to both sides of the reservoir occasionally. If it were mine I would be changing the MC before it killed me. Am I off base, guys?