After pressure testing my lines to track down a leak, I have determined that the return line was the problem. I have dug up the line and found the problem but am a bit stumped on what would be the best way to fix the leak. The leak was caused by a cheap plastic fitting that was threaded on one end and had a barbed end that connected to the black poly pipe. This fitting was extremely brittle and seemed to have been eaten away by years of chlorinated water. This is what connected the poly line to the wall fitting in the pool. Under the pool deck I found a badly rusted elbow that is somehow connected to the wall fitting. At this point I'm wondering if I should just remove the entire wall fitting and put in a new one. I don't think I'll be able to get this rusty elbow off but I can't get anything to thread into the open end of this rusty thing either. I'm not sure if it's because of how badly it's rusted or if I may have damaged the threads when removing the plastic threads that broke off and were stuck inside.
Here are some pictures of the ugliness that I found. If anyone has any suggestions on how they would tackle this, I'd love to hear them.