Tank Venting Question California '06 Sprint

My California '06 Sprint ST tank vents only to the evap canister. It also has a one way valve in the vent line so expansion air can move out of the tank only, but not back in. This creates negative pressure in the tank. Does anyone know how the tank is actually vented? After any running at all, there is a sucking sound when I open the tank to fill up. Doesn't seem to affect the way the bike runs, but I've never had a bike run well with negative pressure in the tank. I'm very curious about this.

The only thing I can imagine is that the tank is designed to run at a certain amount of negative pressure, and there is pressure regulated vent control in the tank cap seal. So when the pressure gets down to a certain level the cap allows some air to flow into the tank. Since the fuel is pumped, not gravity fed, the low pressure may not matter.

I read the workshop manual section and it describes the evap system. I checked all the lines and they are installed correctly, but the manual does not mention pressure differential. Typically when I buy a new bike, I take all the evap stuff off. Has anyone done this?

Thoughts or insight would be appreciated. Thanks.