Soiled wrote:No matter what level of fuel you have the fuel pick up is from the same spot so makes no difference for the dirt etc the fuel filter is there to deal with this.
As for pumping air I dont think thatd be too common as youd notice a big dip in power, funny noises or worst case melted pistons from running lean.
But in a sealed tank with petrol levels going down leaving more space for air to occupy would cause it to be less dense thus a low pressure area is made, opening the cap allows it to equalise.
Only cars Ive had that didnt make this noise were carbied, which a couple sucked air if the fuel tank was low due to the lack of baffling. Made some horrible noises at times!
All this is just opinion though and could be completely wrong haha.
Spot on!! couldn't have said it better myself