I am assuming this is a new tank that was installed when the car was restored or tank replaced. You can add a vent but would have to drop the tank and empty to put in a vent. I would not do this in the car for fear of fire.
I had a buddy do this and he dropped the tank and welded a nipple on the tank. (Chev )
I have also seen some people drill a very small hole in the cap to allow the release of pressure. Not my way of fixing a problem but have seen it done.
I don't know where you can have the tank cap checked but I think there was a device that the old mechanics used to check them similar to what they are doing now to check gas caps for emission testing.