Finding the leak is the hardest part. The pond drops to a certain level, then stops. So, we can assume the leak to be at that level. But, don't assume. My experiences suggests there could be more than a single "leak" especially since soils are suspect. Often, we assume water is leaving from a single source...but that's not "normal" for a leaky pond. Take hundreds of pinhead size leaks, like little veins, add them together and you have a pond which loses an inch each day. I would plan to line the entire pond with clay, if possible. You will get one shot at it, so I would plan to "overkill" if you think it's practical.
Also, since your dam was made of good material, we can assume you have other good material available. Why not use a backhoe to excavate several areas near the pond, see what you have, and try to pulverize and use it? That could save enough money to line the entire pond. If your material is suspect, verify through a compaction and/or plasticity test from a nearby lab. Then, you will know for sure.

Teach a man to grow fish...
He can teach to catch fish...