If the tank is not properly vented, then the air cannot replace the space that the gas you used takes up and creates a pressure back-up, similar to trying to suck liquid out of a straw with your finger on the other end... It can't do it if air cannot get in to replace the volume that the liquid displaces... If this happens, then you need more venting on the tank so air can come in to replace the space the fuel that you used took up...