Possible drain leak or broken pipe?

We have a skimmer and bottom drain both controlled by gate valves. This summer I noticed bubbles in the pool from the supply lines and upon further investigation noticed that there was air in the trap. I replaced the o-ring on the trap lid but no change. Tried opening and closing the two valves and found that with the valve from the bottom drain open and the skimmer valve closed I'm not getting a full draw of water from the bottom drain. Through playing around I've discovered that if I leave the bottom drain valve about half closed it stops drawing air. We've checked the bottom drain and removed the cover - saw no problems or restrictions causing the problem. Water loss on a daily basis is not noticable. All plumbing is 1-1/2" PVC. Anyone have any thoughts or suggestions? I'm hoping it's not a broken pipe....

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