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I realize this is a super old post, but I just cut my old AOS apart to see how it works and there is a way you could non-destructively bench test it. The AOS looks to be basically a vacuum actuated diaphragm valve between the throttle body/intake tube connection (i.e. the connection closest to the round diaphragm body) and the other three connections on the device. One side of the diaphragm is vented to atmosphere and the other is common with the 4 AOS connections. The diaphragm is spring loaded so when there is no vacuum on the intake the spring holds the valve completely open for open flow between the three other connections and the intake tube connection. As the intake tube connection draws heavier vacuum, the pressure differential across the diaphragm increases in the diaphragm valve begins to shut. From posts that I’ve read, it seems the air oil separator stops working when there is a hole or a leak in the diaphragm ( a leak in the diaphragm does not allow pressure differential to counteract the spring, so the valve does not actuate and flow path to the intake tube connection is wide-open). To leak test the diaphragm all you should need to do is plug all but one connection, then blow air at !LOW! pressure into the third. Apply soapy water to the outside of the round diaphragm valve portion while pressure is applied and a leak across the diaphragm should be visible as bubbles around the diaphragm valve casing.
This will not test how well balanced the spring and diaphragm are together, which is probably also important to a properly functioning valve, but it would definitely detect a crack/tear in the diaphragm which seems like the main way these things die.
Posting this because hopefully it will help someone out!
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