Leak in return line...I think

Hi Everyone.

I finally got my water crystal clear and my chemicals on the right track....I figure now is a good time to address the next issue.

I LOVE having a pool. I've added mine to the pool chart (line 176). Last summer I realized I had a leak between my skimmer and pump. So I started by digging up the easy side by the pump (the side that didn't have a 3ft wide concrete patio above it). Of course, not a drop of water in sight. Sooooo I broke down and dug up the spot near the skimmer (down 4ft, over 3ft). I found a cracked elbow right below the skimmer, and built a new one from pieces at Home Depot. Problem solved!

So the pool was working great for a week or two after that issue was resolved. Then I came home from work one day and noticed that my water was 1-2" lower than when I left! So assuming one of my PVC joints didn't hold, I switched my pool to only use the master drain and no skimmer. To my surprise the pool still lost water!

Doing what seemed like the next logical step, I switched it to use just the skimmer....and it still lost water. So, same as last time, I dug up the easy side first (by the pump)....and same results as last time. No Leaks!

After that I decided to try the food coloring trick. I put drops of food coloring allllllll over the pool after my pump had been off for a while. Unfortunately, one of my return jets was sucking it in....

So now I'm not sure what to do...

I noticed the return jet fitting was cracked (Hayward SPX1408B Face Plate I think). So I bought a new one and the matching gasket. I don't know if it's been like that since i bought the house though (2yrs).

Can I change that with the water in the pool, or do I need to drain it down below the return lines?

If it's not the gasket or fitting....then what? I don't have access to dig under the concrete this time. The only way I could get to it is through the concrete. Is there any easier way?

Thanks!