Well...sometimes I don't explain myself very well...sorry. When I typically take my gas cap off to fill up or put gas in, there is always a pressure release that sucks in air (vacuum?). Kind of like the pressure or noise that is created after the car runs and you take off the radiator fill cap although that is air pushing out. In the case of my gas tank, it is air sucking in as if the tank is pressurized. But lately, there is virtually no pressure or air suck noise when I take off the cap. I read on most posts that it should have somewhat of vacuum pressure taking place...hope that is more clear.