Up until now, I've drilled three holes into the white ring and screwed in three screws to extract the ring from the cylinder sleeve. Someone suggested to use compressed air from the top hole.. This worked on two cylinders I have. One came out with ease. With the other, I had to block off some of the passages with a bolt. It seems, the trick is to push in the piston all the way in and then apply air.
Sujit
Sujit