I've had a small coolant leak into a cylinder in the second hand Nascar engine. It will leak about a cup over 5 months and then past the rings to the pan. Oil never milks up. Weird . I think the water just sits below the pan suction fittings since there is about 3/16" step down into pan. Oil heats up and then i get steam from the dry sump reservoir vent till it all boils out of pan.
Back to your issue. Yes seals can hold under pressure or vac but leak in the absence of either.
