You should not be able to blow air back into the fuel tank and pressurize it if the fuel tank is vented properly. I am guessing that the tank is not vented and after you shut the generator off the vacuum created in the fuel tank is pulling the gas back through the line into the tank. I would check to make sure the vent line from the tank is not plugged or make sure the gas cap is allowing air into the tank.
Binnie, Wanda & Yasper (Labradoodle)
1999 Winnebago Chieftain 35U F53 Chassis V10
Towing 2014 Jeep Wrangler Sahara