Air bubbles when closing 3-way valve

[blk_hwk_plt]

When my pump is running and my 3-way valve is in the middle (50/50) position, intake flow is great and no air bubbles come out of the return jets. When I fully close one of return lines (usually the floor drains to maximize the suction of my pool cleaner, but it happens when either is fully closed), the flow of the intake slows and air starts to mix in the pump basket. Then eventually that air will only shoot out of one of my return jets and it's kind of noisy.

Any thoughts what might be causing this?

 

[HermanTX]

It appears it may be a small air leak in the seal of the top for the 3-way valve. When open to 50/50 there may be more unrestricted water flow, so it does not suck in air. When one port is closed, it may draw more suction and the leak becomes evident. Remove the screws that hold the top and lube the oring on the top seal and where the stem goes through the top.