I have the same problem on my XT276. I finally turned the hose pressure up some and while it was running out the vent finally burped and spit a little water out and then spewed air like crazy. It is almost like it is plugged up and I have to pressure it up to make it vent. I checked for kinks in the vent hose but found nothing. Still have no clue why it does this and it takes me 2 hours to fill the tank because more runs out than goes in.