You might be right. All mine used to bulge and I never overprimed, the opposite in fact. But every time the PB goes through a pressure cycle it get slightly weaker. Thats why they ultimately fail as I have found out with pinhole leaks or cracks in the body seam. I've even had a cap crack and then leak. However that's incidental to my point which is modifying a PB or hooking up a compressor to a PB is not safe practice, as is testing with air alone, especially on old or second hand PBs of unknown history. The rubber bands that protect these items are crude and should only be designed to lift and discharge at a rate in line with a slow fermentation process not cope with the flow from a compressor. Anyway I've said my bit now, and more besides. Others can take heed or ignore what I have said, its up to them.