It is really quite simple to trouble shoot a vacuum buildup. Take off the right plastic surrounding the tank...... Follow the line on the top of the filler neck.. It will run back under the plastic, and under the seat. Before in enters under the seat.... There is a little half inch thick aluminum cylinder about 1 inch long. If that one freezes it will not let in air in the tank as fuel is burned. The valve has a little ball inside that rolls around. It can be stuck if debris gets in there. The line then goes further down to the CC..... and even if you have a Y at the canister as the mod/recal dictates.... If the little cylinder is plugged.... the tank will not relief, and a vacuum will be generated. Cure is to either remove the little valve.....or full removal of the CC.