As I stated earlier, the original tank did not do that but it also had a leak around the filler neck next to the tank, Maybe the leak did vent it better. I just took the car for a ride to get gas and it performed well and it did "Whoosh" when I opened the cap.

Your leak allowed the pressure to vent.

Your building pressure caused the leak.

Yes I agree, What to do to stop the pressure build up ?

Have you had your gas cap tested to make sure it is working properly? I believe Jiffy Lube can do it.

FYI,I put a repo tank in my 69 bird.It would build pressure in my garage that faces west in the afternoon.If I opened the"vented"cap it would have a big woosh.I had my 366 NASCAR tunnel port engine in the car,tried to start it one day and it was locked up.Pulled all the plugs and cranked it over,shot fuel everywhere.It had built enough pressure to push the gas past the needle and seats in the factory holley.VERY lucky it did not fire and push a rod threw the unobtainable block.To be safe I drilled a very small hole in the center of the cap.The vent allows air to be pulled in but not out.My factory 350 auto non A/C tank had a vent in the neck.Wish I had just kept it in the car.Tom