Originally Posted by gar
........I checked the vents and all seem clear....
Gar, If each tanks has a vent and you get vacuum from one when the cap is removed, then one or both of the vents must be blocked. I'm guessing you disconnected each vent hose from the hull fitting and blew through it. Did you check that the hull fittings themselves were clear? My fuel vents were plugged with a hard sandy material deposited (so I am told) by industrious wasps!