Does your gas tank have vacum inside ... when your gas tank goes from full to empty does it hiss when you open the gas cap ?
How does gas tank venting work ... does it let air out and in or just out?

 

There is no vacuem inside, never ever, or there should be presure. Just atmo presure. Vent is easy, there is a small tube going parallel to the fieler neck and gets out somewhere in the rubber part in the gas tank door (fieler neack). I think there is a valve there for some other cars, but for audi there is none, I think.

its easy to find it, its the only one there that is odd. check the ruuber thing and inside the neck on top.

 

Slight vacuum on the order of few “inches of water” is possible once you start pulling inches of mercury than you have a problem. On most cars fuel tanks are sealed shut with check valves and so are the fuel lines. I have even seen vent lines controlled by solenoids that open only when the engine is running so the vapors are burned and not vented.

 

Right ... the bently implies thet the gas tank vent hose goes to the charcoal canister ... but my concearn is this ....
I noticed a nasty fuel leak coming from the return fuel line ... near the cat (hard fuel line going through the engine bay, running down the fire wall ... at the bottom part of the firewall it joins another hard fuel line- threaded connection type joint ... that's where it was leaking - the other fuel line runs the whole length of the car joins a soft line which goes in to the tank ... Now the problem is that I was unable to recreate the leak by running the fuel pump (jumped it) .. The conditions were: got a full tank of gas, outside temp 90 deg F, drove 1.5 miles from the gas station to the house, 1.5 hours later discovered the gas leak, have not been able to recreate it yet .... Any ideas as to what might have happened will be really appreciated
could this be caused by impropre fuel tank venting