DIYbanter - pool's main drain is probably the source of leak. any suggestions??

I have an inground pool, and we have had a pretty steady leak the
entire winter. We had a guy come out and give me an initial look, and
he ruled out a leak in the return line since the water keeps dropping.
He suggested that it is either a leak in the pool wall or at the main
drain at the deep end. I have come to think it is the main drain
because the water will not level off. so does anyone know, or have any
insight on what that would cost to repair? He is coming out next
Monday to figure out what else to do. t is a concrete pool. it has
been closed the entire winter, and the filter and pump systems have
not been working. it isnt a leak in the pump and filter because
whether that is running or not, the leak is still present. so any
ideas? The guy suggested plugging the main drain and investing in a
cleaning system? Thoughts?

I have also heard that sometimes there is a valve that regulates air
on the bottom and that may or may not be related? I was first told
that if it was the main drain it may just need some adjustment. who
knows? Does anyone have any good news to add? or anyone with the
same thing happen that just used a robotic cleaner?

THanks