Having built several ponds and fixed a few leaky ones, the full pool water level is probably not where your problem lies, you say that it leaks down to about three fow and sorta stays there, you will probably find your leak is right in that general elevation, a rocky layer is extremely hard if not impossible to seal long term, even if you get it to hold for a while it will more then likely give problems down the road. You might want to consider a pond liner material, expensive, but just a thought.
Good luck, keep us posted
