A13 B210s have a sealed fuel system. There should be no vent line to atmosphere. Instead it vents in the engine compartment to the charcoal cannister.

Perhaps the fuel check valve is bad. This is an inline valve near the fuel tank, in the vapor vent line. It is a one-way valve. You should be able to blow air through (with your mouth) from fuel tank side, but not blow through from engine side. Perhaps it is stuck. Details and test procedure are here: club wiki - Gas Cap

The gas tank cap should be sealed type. It does not let air out. But it does have a vacuum release valve to let air in. Perhaps it has the wrong cap. It should be more or less flat on the outside, with a nubbin on the inside. You should not be able to blow air out throw the cap, but should be able to suck air in through the nubbin.

Maybe the problem is in how the lines are connected in the engine compartment. On the round black charcoal cannister are three lines:
* Hose to carburetor side (vacuum signal line)
* Hose to carburetor side (purge line)
* Hose to fuel tank (vapor vent line)
Ensure they are connected.