Should be pretty straight forward to determine whether you experience vacuum or pressure when you remove the cap.

If the vent is blocked though, it doesn't take much of a pump to suck a fuel tank flat! Think about how many square inches of surface there is on the inside of a fuel tank. Then imagine a pump sucking fuel/fumes out of it. I saw a fuel pump collapse a fuel tank on a ground power unit in the USAF because the vent hole was blocked.

I check the vent opening under the 300D every time I'm under there doing other checks/maintenance.

Wes