Can you shine a little light into that cylinder and see how 'clean' the piston top is?

May sound weird, but I had a bad pinhole spot in the exhast cross-over on my '596 SB castings...it managed to leak enough water at operating temperature over about 5 mins of runtime to completely leave the top nearly spotless. Make no mistake, it was tiny enough of a pinhole to push enough water to just turn into steam but far from a hydrulic lock, enough to clean things right up. By the time I got it home I could tell me coolant level was down a bit.