Sounds like something occluding the vent at the tail of the barrel, if the one in the TD tube is clear. It could be an over-enthusiastic daub of grease (I know I've done that), but if it's standing up to the power of lungs then it's probably something more solid. If the o-ring isn't in its groove, that could cause the problem, but if it's not in the groove it's usually quite in the way of putting the pen back together. I'd take the pen apart (again, and a sigh is appropriate at this point) and run a pin into that tail vent to see if it's clear.
Note that the vent hole in the barrel wall has no involvement in the filling process; the release of the vacuum at the top of the stroke and release of the pressure at the end of the downstroke occur because once the vent groove or vent hole in the filler tube clear or span the O-Ring, air can enter or exit at the back end of the barrel because there is no seal past that of the O-Ring (on the filler knob side).
The vent hole in the barrel is there exclusively to prevent the small pressure increase that could result from screwing down the filler knob after filling the pen; as the knob is screwed in, it can compress the small volume of air that lies between the knob and the O-ring, and this can blow past the O-Ring, causing a slight compression of the sac and the ejection of some ink. The vent hole in the barrel is located between the knob's lowest position and the O-RIng, and thus vents that space to the atmosphere.