Finding leaks is difficult and uncertain at pool side; doing it via the internet is . . . almost certain to fail.

I'm sorry, but I have never come close to finding a way to help much with leaks via the Forum.

HOWEVER, the valve process you describe suggests that you have a broken main drain line OUTSIDE the pool, below water level (since it leaks), but not very far below (since sucks air). Some leak testing companies can close the MD line at both ends, pressurize it, and locate the leak -- approximately -- via sound. Then, the digging begins . . .