MG T-ABC

Am I correct in thinking that the TC petrol cap should be vented. I'm just at the stage of adding petrol to the new tank and finding that pressure is building within the tank as the fuel evaporates. If the cap isn't vented then how is the system vented.

The TC petrol cap does vent. There is an excellent Tech Tip on the Doug Pelton's From The Frame Up web site showing exactly how the cap is assembled. You will notice that if it is done as per instructions the cap will vent.

Mine vents so well that I must take care to not fill the fuel tank too high, or else it will slosh right out!