AO Platinum Member
Join Date: Feb 2002
Location: The land that Time forgot
Re: petrol cap
You want a "release of air", as the petrol tank must be closed to the atmosphere for evaporative emissions reasons (i.e. fuel isn't allowed to evaporate into the atmosphere via a tank vent or hole in the filler cap any more).
Inevitably, as the fuel level drops you get a partial vacuum in the filler neck. The "release of air" is actually air rushing in, not out.