Losing prime on a 60+ year old pool

[PoolDoc]

I have a 1950's 40,000 gal gunite pool with on two skimmers (bottom drain closed off).

With steel pipes, right? I'm amazed you're still able to operate with those.

I had the system checked and was told that there were no leaks in the pump/filter equipment so he said it had to be in my 'old lines'.

Sounds reasonable.

I just can't see how it could be sucking air from a line that's several feet underground?

Happens all the time.

You've got a leak detector company who guarantees they'll find THE leak (not just a leak or some leaks) for $500? Lucky you! I mean that. Many will just look for leaks, for that price.

Sorry, but if you've made it almost 70 years with original metal piping, it's not really unreasonable to have to re-plumb the pool. Sorry.