Somewhere in the forum is a picture of a cut-open fuel tank so you can see where the internal vent hose connects to the filler opening ring, you'll want to see that for clarity as the mechanics don't work as you might expect. I looked for it, no luck, will try a few more minutes but then its off to bed for me lol and I'll look more tomorrow.
The top fuel tank has two hoses to the under-seat tank.
The hose for the left side dry break sends fuel to the under-seat tank, and can get air trapped in it if the under-seat dry break is loose or kinked.
The hose for the under-seat dry break is the air vent line and it goes in the tank and into a steel tube that travels to the fuel filler hole. In that area is a small red rubber piece that is part of the venting system when the fuel door is closed.
So, be sure the dry breaks are both clicked all the way closed, and make sure the little red rubber piece exists inside the filler hole.
Also, there are two small rubber hoses off the front left-side of the main fuel tank, next to the motorcycle's frame neck. Be sure they are not plugged crushed or twisted. These rarely cause vacuum inside the main tank, but making sure they are in good conditon is helpful along the way.