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They need a constant vaccum it doesn't matter from where. They have a rubber valve/bladder inside that seals off under vaccum only allowing oil to go into the engine. If not it is possible it will go out the oil inj. onto the top of the engine, or into the intake. This can be a good source of oil in an IC/turbo if the rubber has torn and has a hole in it.
I took one apart a few years ago to see what was inside. I have no idea why they aren't just sealed up completely but I'd guess it has to do with some kind of pressure relief when the engine is cut off. I may some pics somewhere I'll see if I can find them.
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