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Hydraulics sometimes bleed back internally so a 'leak" would not be obvious.

Take the cylinder out and have it rebuilt by a hydraulic shop. That would by my first step. If that does not fix the problem, then go to the valve and replace it. Not uncommon for valve seals to leak, so if your really handy taking it apart and changing the "O" rings could cure the problem.

thank you. How do I find it? I see nothing wet.

Wrong! It has a leak. It is just an internal leak. Possibly àthe piston or the check valve or the solenoid. But it DOES have a leak