Pool and Spa Forum: Pool and Spa Forum -> Leak - Help!

I have read several solutions to various problems for leakage in the inground liner pools, but nobody has touched the solution for a what to do if the leak is in the main drain.
I have done all the plugging & unplugging of inlet/outlet. Went away for 9 days with only the main drain open and my water level dropped 14".
The water level drops only when the pool filter is running.
I dug up the lines that are about a 8" underground, no leaks. Pressure is good on pump, no excessive ground water anywhere. Had a diver out last year to find & fix any holes in liner and he repaired two.
Can anyone help me?