I'm onboard with the filter being a great probabilty of issue, but only because I've seen soo many people with them having problems...
now, in reality, I'm putting my vote in that the petcock itself is not functioning....
if the poppet o ring fails, you get fuel in contact with the diaphragm.... and it comes out the weep hole to verify its compromised.... you note it is dripping....
if fuel is in the diaphragm area, which it should not be, then the diaphragm is now swollen and warped, and stretched due to softening, this in turn results in it not opening the poppet fully, as the rubber is now a baloon, not able to pull the poppet open, with the applied vacume it normally sees....it should be stretched tight like a drum to function... pull it apart and look at it, I bet its not tight at all from fuel contamination....
when you switch to prime, you get fuel.... that has to tell you something.
hey, they wear out....just a normal thing.