I pressure tested a main drain line inside a fountain. it wouldn't hold pressure, so I did a sonar test to spot the leak. Once I heard the leak, I cut the bottom of the fountain out, but the leak wasn't there. I went back to sonar again, and heard it a little further down the line. I excavated there, but again the leak didn't show. there's maybe 20 - 30 ' of pipe before it hits the equipment. I still haven't seen any evedence of a leak. any advice?

The leak ended up being a loose glue joint under the rim of the fountain. I was able to excavate under it and get it repaired.