I did similar with my 3c only I used a Ducati Cap that used an O ring to seal it. I used a 6 mm separate breather fixed high and on the right side of the tank to get max capasity, and then it got interesting. If I filled the tank high enough to submerge the end of that breather, it would self sypon till it lost enough fuel till the breather wasn't covered. Bloody annoying. The breather hung into the tank approximately 15mm. I tried a one-way mx style hose and valve connected to it, nothing stopped the syphon till I drilled a tiny hole in that Ducati cap and broke the vacuum. That's why I asked about the breather, I chased this for a good while, tank vacuums are an odd thing.