Go to Rock Auto and look at the pictures of the Gates gas cap. You'll see that the underside of the cap has a plastic vent on it with 5 holes.
It's' held in place with two metal tabs . Those can be bent up to remove the plastic assembly. Inside you'll find a valve and springs that are supposed to allow the cap to equalize pressure inside the tank.
As usual, the replacement manufacturers haven't figured out how to make simple things function properly so the valve ends up sticking closed and creating a vacuum inside the tank.
Remove the little valve from inside the plastic and reassemble the cap. Works perfectly after that.
