Jason
Assuming your running the cap pictured, vented/anti-surge. It is normal for the tank to run at a positive pressure. These caps are not directly vented to atmosphere. There workin against a very light spring pressure. And the lower the fuel level in your tank the larger volume of are will be displaced. If you take the cap off and lightly blow through the small hole on the back, you should notice a slight resistance to flow before it vents.
If the cap were vented stright to atmosphere gas would come out during acceleration with a full tank.
