The flow control hole is only 1/16" in diameter, and it can get plugged up, but it's also possible you were a little impatient and opened up the lid before the oil had a chance to drain down to the level of the hole (about halfway down the center standpipe). Let it sit a couple of hours after running the engine before opening the lid, and the oil level in the filter should be 2 or 3 inches below the top. The orfice is most commonly found at a level even with the inlet fitting hole in the side of the canister where it can be cleaned out by sticking a piece of thin wire through the hole. There is no check valve at the inlet fitting, so it would be unlikely that the canister could retain pressure for more than a few seconds after engine shutdown.
Jerry
