Fuel tank vent

[Chrome or rubber]

Regarding a 79 roadster:
Should the fuel tank be vented and is that via a separate vent line or simply a vent hole in the cap?
When I unlock the fuel cap there is a marked hiss, caused by tank/atmosphere pressure differential.

Thanks

[Bumpa]

Is your car an import that would have had emission controls on the engine? Those had a sealed cap and alternative breathers through a charcoal canister. If it is a standard UK car I believe it must have a vented cap as there will be no other way for air to replace used fuel. The hiss you describe sounds like the cap is not venting properly. It must be venting a little or you would have suffered fuel starvation after driving for a while. I'm told the draw of the pump on a sealed tank can actually collapse the tank a bit. Get a new vented cap, or try and sort out the one you've got!