You sure the other line is for pressure? - maybe it's just a vent, or return for excess fuel from the carb pump.

I've never heard of a pressurised petrol ( gas) tank. - what does the "pressure" line connect to?

Is it not more likely that the vacuum in the tank is the result of the fuel being drawn from an unvented tank?

I can't see how it is pressurised through the carb, perhaps I'm a bit dumb, so If anyone wants to correct me - that's ok!