Air in suction line.

I have air bubbles coming out my outlet in the pool. I have two lines coming to the pump and 3 way valve. One from the bottom of the pool and one from the topside at the strainer basket. If I shut the valve on the line from the bottom, the bubbles stop so I think I have isolated the problem line. Does anyone have a good method of trying to fine the leak location? Could air come in thru the valve stem? I'm going to assume that the leak is at one of 3 joints that are above ground. If not, I'm out of luck. When I turn the pump off I don't see any water leaking out at the joints.

Though air can get in through the valve stem, it would do it regardless as the what position the handle was turned, so it sounds like the low suction line is the culprit. What I would do is turn off the skimmer line and slowly but continuously dribble water on any suspect connection and watch to see if the bubbles lessen or stop, if they do, you've found a leak. I would check any threaded connection first.