cb100,
It sounds like you are doing everything right. Are you digging with your own excavator/backhoe? If so, that is a big plus.
It is hard to do geology over the internet, but if water is running pretty steadily from the walls of your pit and into your sump, then you are PROBABLY still digging in muck, and NOT the clay seal.
Keep grabbing some samples from your deepest scoop every time you dig a little more in your sump pit. I expect that at some point you will reach a good clay layer. (Keep performing the "clay ball" test.)
Cleaning out the muck is certainly an expected step for a 48-year-old pond. Once you get the muck out, then you can evaluate your clay layer. Hopefully, you will find some thin spots that are likely candidates for your leak(s). At that point, you can evaluate bringing in more clay from adjacent locations or using the SoilFloc to seal up your weak clay areas.
**If you use the search function, there are lots of discussions of sealing off ponds with the SoilFloc. TJ is DEFINITELY one of the experts for advice on that subject!
