I once had an issue with venting not working/plugged on one of my 69s. The vacuum in the tank would become so great that the bottom of the tank would "oil can" and have a "whoosh" of air into the tank when removing the cap. Eventually causing the tank to crack along one of the stamped reinforcement ribs on the bottom of tank. I now know to confirm that the venting is working correctly. Had to replace that tank.😢