Originally Posted by motion30
Make sure the vent hose does not have a low spot it needs to slope from the fitting to the tank A low spot will hold fuel and effectly plug the vent When you blew into it it cleared the fuel out of the hose
This is what I would suspect from the OP's description.
The vent must run uphill from the tank to the opening on the hull with no dips. Sometimes an installer will put an upward loop near the hull fitting to keep water out. That's OK, but no downward loops.