Brillo,
Welcome to the forum. I a-s-s-ume (Grrrr) when you say you close the bottom drain valve halfway that the skimmer is fully open? Anyhow, seems like when you get a good suction from the bottom drain you get air. If you get air you have a leak that may not be present under lower suction conditions. All I can suggest is to do an accurate level check and see if it shows a leak. In a quiet pool you should be able to make level measurements to 1/16". Using a ruler you can measure the water level at some known spot like a location on a step, skimmer housing or anything else. See if that shows anything. Not conclusive but another clue.
Al
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