Gas cap under pressure when removed

When removing gas cap after a drive I found the gas cap came off with quite some pressure.
I have renewed expansion tanks under cowl and in the front fender. Lines from the engine to the charcoal filter are clear when I blow air through them. I can blow air through the large charcoal filter tubes. The small charcoal filter tube does not blow air easily through it.
All clear lines from gas tank to expansion tanks are new. Anyone have a clue why the gas cap would come off with force?
My car is a 1971 911T running Weber carbs.
Thanks
John

Keep looking. Sounds like your tank vent has restrictions somewhere. If the tank is properly vented, it can’t build up pressure.

This diagram is for a 1970, but perhaps it could help.

Attachment 503994

Sounds to me like the vent on the tank itself is blocked. Might take the gas cap off and then try to blow some compressed air through the tank vent. It is is plugged you can run some piano wire into the vent and try to break thorough the blockage. (A little trick Richard Humphrey uses)

Best of luck,

Brad