Here is an update on my problem. Maybe it will help somebody. After lowering the water below the returns the pool stopped loosing water (with the pump off) , so that ruled out a leak in the liner below water level and ruled out a leak in the main drain. I tried testing the lines with air pressure, but the test plugs did not seal well enough. I then made a simple test setup to test the returns. I took a short piece of 1-1/2" threaded pipe and connected and elbow and about a 2-foot long piece of pipe. I screwed this into one return fitting and plugged the other. I then filled the pipe with water and timed how long it took to drain from the pipe. The pipe drained in about 20 seconds. After leaving it sit for awhile I filled the pipe again. It did not take much water, which indicated that the leak was probably close to the pool. I did the same thing with the second return. With that return the water leaked from the standpipe faster than I could fill it.

I decided to dig down to the return on that side of the pool. My concrete deck is only 3 feet wide side it was not real hard to dig down and over. The interesting think it that on the other side the soil was saturated. On this side the soil (clay) was hard and dry until I got within about 6 inches of the return pipe. Then it became very wet. Once I exposed a bit of the pipe I took a hose and placed in against the return with the eyeball fitting in place. I just held it in place so the pipes were not pressurized to the full pressure from the hose. After doing this for a short time I could see water flowing into the hole I dug from above (I had not exposed the area around the elbow). That is as far as I have gotten. I am certain at this point that it is leaking at the elbow in the return pipe. It is getting too cold to pull the liner back so I am going to wait until spring to fix it. It is just good to know where the leak is.

Another thing. I had dug down to the pipes adjacent to the pump since I thought that settlement of the equipment slab may have caused a problem there. If I could not figure out which line was leaking I had considered cutting one of the return lines at that area and installing a valve so I could isolate the pipes. I did not need to do this, but it would have been a relatively easy way of isolating one line at a time. I was thinking of cutting the bottom off of a plastic pool chemical bucket and burying this around the valve with the lid several inches below grade. That way I could access the valve relatively easily in the future if needed. I could also have piped the valve about grade, but I assumed that I would not really need to access it in the future.