As far as I know, a little pressure in the fuel tank is an OK thing.
Depending on the amount of fuel in the tank and the ambient
temperature, there will always be some vapor pressure there. You
would see the most in an almost empty tank on a hot day. My '64 B
has a brand new vented cap, no vapor canister, and still builds up
some pressure. The key to tank venting is to not create a vacuum as
the fuel is pumped out. A powerful enough pump would cause the tank
to collapse without atmospheric or vapor pressure behind it.
Chris Attias
'64 MGB
'84 Alfa Romeo GTV-6
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