Good thought , but it Wouldn't make sense for the check valve. It only stops oil from flowing away from the fuel pump, not into it. Now I've been giving this some thought, and it would make more sense that it's the fuel pump not the oil pump allowing this to happen. Since the oil pump, even at idle, never completely closes, oil could gravity feed through the oil pump. But the fuel pump should stop the flow of oil and fuel when it's not being operated right? This assumption is based on the fact that when I take my fuel lines off to clean carbs my whole tank doesn't siphon out. Does this sound right? Lol