I don't think this subject is straightforward. All my MGBs had a fuel tank vacuum way back in the seventies. I believe that the tanks were designed to vent through the cap, and I suspect current Morgans may be similar. On my MGBs I made a small hacksaw cut in the filler neck to allow better venting, but I think the idea was originally to stop fumes venting from the tank.
My Roadster doesn't seem to have a tank vacuum, but my Plus 4 does have this when I go from a full tank to a quarter tank, then release the cap. I don't really worry about it.
Our wedding photographer bought a new Lancia Beta, and within a week, the vacuum had sucked the tank so far in that it would only hold a gallon........
