Welcome to the forums JB!
If I am understanding you correctly, the pond has never filled up over the course of 14 years?...
I suspect that a pond will fill up to, or better put, drains down to the lower section of the breech. If the pond has typically had, for example, only one foot of water in it then the leak has to be low in the hole (somewhere along the shoreline of that one foot of water and possibly well above it). If the entire pond bottom is absorbing the water instead of one leak channeling it elswhere then the clay content is weak.
Just my thoughts as I am no pond builder, but I would be curious to know if it holds water at all under normal conditions (no pumping) and at what depth will it maintain.
Pond banks (and beyond due to capillary action)will absorb water at the shore line as the water depth increases, but if the pond has not filled over the course of years, the water is leaking/soaking out somewhere.
