Wow, lots of talk about this little matter. I just went through it myself on 08 Fat Boy. Being kind of remote north of Saigon I only found oversize o-rings. Spent a sunday afternoon sanding them down to make one that finally fit. Thought it was holding, but overnight it started to leak again. Into the city a bike shop had some that worked. Not exact but holding.
Singapore shop only sells complete check valve with the teflon fuel line $130. Same story from HD in Texas. So got to do what I have to do. Will change out on arrival.
Anything with moving parts is going to wear, but this is a matter HD should address. Either improve it, or make these little rings available.