Well this has been intresting.....my '62 had an occasional fuel seepage from the sender. A few days ago the leak suddenly became a flood and so I went under and fixed the kludge that the PO's mechanic had done on the sender gasket. Drove slowly a mile or so today and suddenly "POOOOOM" from, under the back of the car....."Geez did the the whole tank fall out?" Stopped and looked under and the tank was partly sucked in on the bottom....great. Pulled off the gas cap and 'sssssssssssssssssssssst" a long inflow of air occured; I mean it really had a lot of suction in the tank; well I guess the sender's seal is now good! Fortunately the tank popped back out almost 100%. So it appears that the tank cap was not venting and that was the likely cause of the sudden severe sender area leak: it sucked the kludged-in sender gasket out of whack. Went to all 5 local auto parts stores looking for a vented cap; only Autozone had one and it was identical to the one on the car that was not venting. Sucked on it by mouth and it seemed to vent OK so put it on the car and drive and checked and it was fine; no vacuum build up in the tank anymore. I never had gas cap 'go bad' except for an old gasket cracking; but it appears this one did. The bad cap now seems to vent but it sure was stuck before. I don't trust the new identical one from Autozone to not do the same, so I am getting a new one from NAPA tomorrow and may get a STANT replacement too and pick what looks to be the best. Seems like a new experience every week.....LOL Edit to add: The new NAPA cap (made by Stant) fits better, is all metal, and has beeter ariflow. Pix below shows the Dura-garbage cap on the left and the NAPA/Stant on the right. Who would think something so simple could be made well or poorly?