Leaking Car Petrol Tank

Excess fuel is returned to the tank via a separate pipe. Or rather does on older vehicles. Some more recent ones control the actual pump output only to supply fuel as needed. Older ones use a separate regulator. But even some even older carb cars returned fuel to the tank. Circulating it helps prevent heat evaporation cause in the engine compartment. However, pressurizing the tank would make it *more* likely to leak, if only fumes past the cap seal. They are more normally under a slight vacuum.

While fuel tanks shouldn't be pressurised you can occasionally get an outrush of vapour that comes from agitating the fuel (and heating by its surroundings)

Most (all?) petrol tanks are now 'sealed' and vented via vapour recovery canisters filled with charcoal and a vacuum line to the inlet manifold controlled by a solenoid valve.

Quite. Which produces a slight vacuum, as I said. If it is pressurized, something is wrong.