It sounds like the rollover valve to me. I had the same problem after I had the tank off. I decided to route the tank vent lines a little differently to tidy thing up and didn’t fully understand the point of the roller over valve. I left it sitting at a 45* angle and it sealed off the vent line, needless to say the bike ran like crap as the tank got low but ran fine one the tank was vented. Once I set the valve back vertical the problem was resolved. If you pull the valve and hose and blow through it and tip the valve you will feel just how sensitive it is and is clearly designed to block flow in both directions if it is tipped even slightly off vertical. I suspect it is also susceptible to failure due to its sensitivity.
If it ever causes me any problem again it will be removed. My dirt bike leaks gas every time it’s in a horizontal position and this is a frequent occurrence; I have yet to burst into flames.
