Well, Im not sure if the injector pump could be leaking internally, allowing oil to bypass it, but that surely sounds like a possibility. I have never taken one apart before, though.
Are you smoking like heck while running? My guess is that the oil is entering the crankcase down in cylinder 3 and getting drawn into the combustion chamber while running.
You can also blow through your old check valve, I believe if you blow through it you will only get air flow in one direction.
I have also seen the tank pressurize itself on hot days and force oil down past the check valve.
Another thing you could do is pull the oil hose off of the injector pump and put it in a bottle and see (while the check valve is installed) if the oil self drains into the bottle.