[N4Fun]

Since I removed my EVAP and plugged the hose to the tank I've had a good amount of vacuum created in the tank, it's very noticeable when I open the gas cap to add gas. I have to get my fingernails under the edge of cap to pry it open. So I bought one way breather valves for dirt bike gas caps and replaced the plug in the hose with it. When I opened the hose it took a while for it to equalize pressure in the tank. See pics below. I did test if the one way function actually worked with a syringe and on these cheap valves it did not, water squirted right out of the vent holes. My feeling is that the system is built so little or no gas actually gets out through the vent, they wouldn't want fuel in the EVAP canister I would think. So I went ahead and installed and I'll see how it goes. At least I don't have to worry about fuel starvation.

 

[N4Fun]

There's no need to plug the hose to the tank. That will possibly be why your getting increased vacuum pressure in there. I left mine open and I've not had any issues.

In that case I paid $6 for a pretty red ornament to hang off my hose ?

 

[N4Fun]

I’d remove the valve or replace it with a long hose if you want something there, because I’m pretty sure it needs to breathe both ways. I left a cap over that little nipple on the tank while I was doing some tank-off work - the sun coming in through my open garage door hitting the tank on the floor heated it enough to build pressure with nowhere for it to go. The result was a big pool of fuel on my garage floor.

The valve breathes both ways, its supposed to stop fuel from coming but allow air both ways.

 

[N4Fun]

Remember its a box not a can, specifically its a rectangular box not a cylinder and is black with a black band around it so easy to miss if you're peeking thru the fairing and don't have good light...