I don't think that is true as the fully depressed lever compress the spring and creates the vacuum to draw in fuel. No pressure as the cam, lever hold diaphragm up. Plus, non of these mechanical pumps have perfect check valves, in fact usually there is a bleed hole in check valve button for anti-perc purposes. So pressure would bleed back to tank.

I am really confused about the non vented cap also. Unless I am missing something, must be a lot of folks with plugged tank vents and collapsed fuel tanks from the suction/vacuum pulling them inward.

I also think we need a lot more information on where fuel is specifically leaking out from to flood all the cylinders. If the pump overcame seat force and kept needle open the fuel should spill out the vents on the top? Is the top of the carb wet?

If a venture gasket was missing you could wind up with normal fuel level spilling out raw fuel into venture. Or if the bleeds were plugged causing a syphon action that would continue to leak fuel out venturi nozzle. It is even possible an accerator pump nozzle can drip from syphon effect if the closed design.

Having said all that, the fact that two different carbs do this, does lead you to believe something else is at issue unrelated to carb.