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My mistake, there is no anti-drainback mechanism in the passages from the pump to the filter, I got it mixed up. That valve is in some (most?) filters to prevent the oil from draining back out of the filter when the engine is not running. This is important because a lot of engine wear happens when starting the car, so you want as much oil as possible stored past the pump to get to the main bearings (also related to why you should fill up the new filter).
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