Here's my reply to a similar post not too long ago:
That cylinder should have a threaded gland. It uses U-cups for both the piston and gland seals. It is necessary to remove the elbow from the base end port and insert as large a drift punch as will fit into the port AND a drilling in the side of the round piston nut. Once the nut is trapped, the rod is rotated, loosening the piston and relieving compression on the U-cup piston seals. It may not be absolutely necessary to loosen the piston to get the rod out of the bore, but the piston MUST be installed loose and tightened in place at reassembly. And loosening the piston as described is a good way to free it without damage to the O.D.
Hope this helps