Pool leaking and air in pump only when main drain suction is

[dbuxton]

I have a 15000 gallon in-ground pool that was installed 7 years ago. Recently I have noticed it loosing about 1" of water per night (pump is off). When the pump is on I only get air bubbles from the main drain suction side, if I change the valve to suck only from the skimmer it is fine. If the valve is moved to half skimmer and half main drain or all main drain the pump gets air in it. I pulled out my SCUBA gear and tried leak testing all of the fittings, light fixture, etc.. using food coloring and a syringe but could not find a leak. I couldn't find plugs to fit the drain pipes so I removed the main drain cover and stuck two tennis balls (not perfect but its what I had on hand) in the pipe to temporarily plug them and one in the skimmer pipe overnight and it still leaked??? I suspect that the leak is associated with the main drain since that is where air is getting into the pump from, is there any other way air can get to the suction side of the pump??? I did check all of the gasket on the pump and they were fine. My pool also has self cleaning jets on the bottom, can air enter the system through those in anyway?? Any suggestions of other things I can do before hiring someone to pressure check the system? Thanks

[ChuckGeo]

A leak in a suction line can show air in pump when the line is in operation and leak water when the line is off. Sounds like you may have a leak in the main drain line. You can dye test the line by valving it off and stuffing a sponge in the main drain suction ports, then insert a piece of 1/4" tubing past the sponge in one main drain, leaving the end of the tube exposed in the sump. Dye test at the exposed end of the tube - if the line is leaking you'll get dye draw at the tube. If so, you'll need to get the line pressure tested to locate the leak. Alternatively you can plug the suctions in the sumps, valve off the main drain and run the pool on skimmers only.