Is the pneumatic cylinder leaking air? Just so you don't go through all that trouble of changing it for nothing because it is pretty rare that they go bad. Have you verified that the air solenoid is functioning? If the air solenoid is not functioning then does it get power when the open/close switch is pressed. If no power then a fuse may have gone. If the solenoid does get power and does work then is there air coming out of the small air regulator that controls the air pressure to the pneumatic cylinder?
"Gunner" USN Retired, Airdale
2004 Monaco Signature 44' Conquest. Detroit 60