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In my Daimler Double Six (1988) I find that the fuel tanks pressurise after a few miles. If I open a fuel filler cap, there is a big release of air.
I find the Haynes manual description of the tank and fuel venting system just impossible to trace in the car, including the carbon filter and recirculation of the fuel.

Is this pressurisation normal? I have had the car some 17 years and it has always done it and there seems so to be no negative consequences but it just does not seem right.

I would certainly welcome some recommendations on this and maybe someone (perhaps Bob Bates) could could do a technical article on the fuel systems of the V12s in this respect.

Are you sure it's an outrush of air? (It can be, but an in-rush is also possible.)

I am not sure you will find a carbon cannister if you are in UK, my '84 certainly doesn't have one. If you are in the US or Canada then, yes, there is one, front right wheel well may be the place to start looking.

There's a vent system which starts with an almost microscopic hole in the filler neck. This feeds to a pipe which runs down inside the wing. Exactly where it goes next is hard to discern from the parts cat but on my car it emerges near the diff cage. Checking this is unblocked might be a first step?

I agree with Al in that the small bore breather pipe can become blocked by road dirt. I use an old nylon electric aerial drive cable to clear mine from time to time. It's just the right size and brilliant for clearing quite a length of the pipe. Mike.

Thanks guys, I will go hunting the vent pipes. Been at the Silverstone event, hence no action yet. Peter