Definitely vacuum, not pressure buildup. Sorry if I was unclear. I took off the filler neck and checked the checkball and it rolls freely when upended. Vent line is clear. I installed all new fuel lines/return lines, but have not tried to blow air through them to see if any are clogged. Could that be the problem? Will the charcoal canister being plugged cause this? The thing is, if loosening the cap relieves the vacuum pressure, you'd think the cap was the problem, right? Or is something else causing excessive vacuum in the fuel system/tank?