Hayward Vac problems in Round pool

[tabor]

There is no "back half". It's a hole in the concrete where the water comes out. The pool is at least 30 years old.

[PoolDoc]

There has to be piping in the hole somewhere; concrete as it is installed in pools will erode from continuous flow. Most likely, if you look back in the hole with a light, you'll see the piping.

This is sub-standard construction, so there's no telling how it was done. Without photos of the opening AND down into the hole, I'd have no idea how to correct your problem.