I believe that vent barbed fitting should be on the top rear of the right center case. Are you missing that pressed in fitting? and just have a hole?
If you just have a hole or just the nub/barbed fitting, It may just be a bit too low and letting some oil out; Or there is too much oil or some blow by on the piston rings.
It's just like vacuum tube. If one of the guys doesn't have one (Shortline probably does) then I've bought preformed hoses for other applications at Autozone as they just hang them on a rack. The hose comes off, Then it "T's" off up high and routes down below the swinger.
There isn't a true check valve, just a pair of lips at the lowest point to prevent crap from entering it.
Service Honda or any of them have a good pic of how it routes usually.........
All our government does is distract us while they steal from us, misspend our tax $ and ruin our country