Without seeing it or asking 10 mill questions I'd start by putting a level on the piston. I have seen the. Shift before and cause everything from a hard knock to pissing out oil. The next thing is check is how glazed over or shiney the piston is. If it's so smooth it looks and feels like it's chromed that can also cause it to start leaking after a while. The piston should have very small cross hatches that carry a small amount of oil to lubricate the seal. I've had very old pistons that were super smooth that wouldnt seal, especially in the down direction. After inspecting the piston and finding no real problems we sanded the entire piston. (7 stop 6" piston) It didn't stop the leaking 100% but it cut it back by about 75%.
