think about what ya typed.
you mostly solved your own issue if you will simply think.
if the primer is sucking flat it means the fuel pump is sucking like crazy.
it also means the fuel restriction is between the primer and the liquid level in the tank.
see how simple this stuff is

you may also wish to change the medium pressure filter and clean the VST filter as they dont tolerate trash well.
they have nothing to do with the primer but they dont tolerate trash.