I think some sort of releasing fluid is going to be needed, then the cylinder can be placed in a padded vice and the piston tapped out via the cocking slot, once the cocking rod is out the back you can take the cylinder out the vice and put the rod in the vice and try pulling and twisting the cylinder away from the piston, they can be right buggers sometimes, the piston head in my Challenger got stuck once and I had to get a very thin and long screw driver from the bay of evil and stick it down the transfer port and beat it out.
Pete