[Keith True]

It sounds like you are using a screen.I had something like this happen with one design of screen,but I don't remember if it was the screen,perforated,or slotted opening type.As gas is being pumped in through the screen,the air is not allowed out because the gas rushing in is holding it back.As soon as the gas stops flowing it burps itself out.It's really just a little bit of gas that overflows,but it looks like a lot on your car.