Well, just think about the vacuum source. It is supposed to be from the base of the carb which is constant vacuum when the engine is running. I haven't thought about what the valve does when you step on the gas and the vacuum goes away. There is no spring inside to make the plunger do anything. If the blow by pressure is enough to unseat the valve then the vacuum sucks it into the base of the carb and the air stream. (I would imagine) A bad pcv, carboned, rusty, or the like, wont seal at low engine speeds and could cause a vacuum leak. If your engine is running borderline lean, I could see where it could lean it out even more at highway speeds when the blow by pressure is high. Just thinking out loud.