I have driven my 1980 Camaro Z28 very little since I got it back from being restored three weeks ago tomorrow. On the way from the exhaust shop to the transmission shop I stopped to add two gallons of gas. When I unscrewed the gas cap a loud swoooosh noise was made and a good deal of air rushed out of the tank.

One the way home from the transmission shop, I stopped to add more gas and the same swooosh noise was made as well as a large amount of air came out under pressure just like it did before. It was about a month between adding gas.

What could be causing this pressure to build up in the gas tank? What can be done to fix the problem? If it matters, I am using the car's original fuel tank and carburator. The car has the non-California emissions equipment. It was sold new in Alabama.

What year car?
70 cars without Califormia emissions used a vented fuel system. The gas tank cap was the vent, and if the system builds pressure, it is usually a bad cap.
70 cars with California emissions and 71 and later cars used an unvented system, so the venting was handled through the charcoal cannister. Problems in these systems usually mean something wrong with the charcoal cannister or the hoses to them routed incorrectly.

What year car?
70 cars without Califormia emissions used a vented fuel system. The gas tank cap was the vent, and if the system builds pressure, it is usually a bad cap.
70 cars with California emissions and 71 and later cars used an unvented system, so the venting was handled through the charcoal cannister. Problems in these systems usually mean something wrong with the charcoal cannister or the hoses to them routed incorrectly.

Gary,

It is a 1980 with non-California emissions. The car was sold new in Alabama.

I will have my mechanic check the hoses because I think the charcoal cannister was removed during the car's restoration.