How is it that the cylinder slowly shrinks a few centimeters after reaching the retraction limit?

After eliminating the air leakage of cylinder and solenoid valve, it is likely that the buffer is too tight.

You can find a screw with a small hexagon on the cylinder head cover. Observe the screw point carefully and turn it counterclockwise a little.

The cylinder is a cylindrical metal part, which guides the piston to do linear reciprocating motion in the cylinder.