Suction side leak

Just started up my pool. When I pull water from the main drain and skimmers, little if any air gets into the pump. If I start the pressure cleaner or turn of 1 or both skimmers on, the pump pulls a lot of air. I tried replacing the pump basket O-ring as well as the pump drain O-rings. This did not help. Pool is about 7 years old. Any suggestions for a diagnostic plan?
 
Some progress. Rebuilt all valves, replaced all drain plugs and replaced pump main seal. Leak appears to be in the drain line. If I reduce the flow from the main drain, I can get the pressure cleaner to work without sucking air. Diver is coming next week to pressure test the drain line. No wet spots anywhere I can see and the pool is still losing about 1/4 inch of water overnight with pump off but no loss with pump on.
 
A suction side air leak can be underground but almost NEVER is. Before I hired a diver I would re-inspect all connections of main drain line that are above ground.....starting by checking the impeller and working out until it goes underground.
 
Thanks for the advice. The part that makes me think the leak is underground is that the pool only leaks when the pump is off. The rate is about 1/4 inch per day and there are no visible wet spots above ground. Pool leaked down to about 13 inches below the first tile over the winter. I added about 6 inches of water and it still went down to the 13 inch mark before the pool was opened and the cover removed. What do you think?
 
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