Wilsons Sandbox

Good evening MGers Rather odd experience yesterday - out in the sun yesterday afternoon in Suffolk's Indian Summer working on the TC petrol started to poor from the petrol pump outlet - currently the Carbs are disconnected - I flipped the petrol tank cap and with a rush of air pressure was released. Should the internals of the tank cap have some sort of vent hole? Mine doesn't nor had an old one from a previous TC I looked at. Am I missing something? Regards Geoff TC 6996 Sunny and warm Suffolk