Porsche: 911 Fuel Tank Breather - Air Sucked into Tank!

I have a 1980 911 SC Targa and I am finding that when I take the fuel
cap off to refuel, there is a large amount of air being sucked into the

fuel tank. I assume there is some sort of breather valve somewhere
which would normally allow air in and no fuel vapour out? Where exactly

is this? Is it part of the fuelling pipework between the tank and the
filler cap?
I assume that something is just clogged, where exactly should I be
looking?
Cheers
Murray