Right, so there isn't a problem with pressure.
You have checked the 3 "O" rings around the pack body, its essential these are sound, if so you will have to strip the pack, start with removing the big circlip and checking the cuff seal on the piston, I would be surprised if the plates are bad if you haven't let them slip for ages.
If these are OK then you will have to press the shaft out of the ta body, on this shaft are 2 metal piston rings whose ends hook together, these breaking are one of the common causes of sudden pack failure, don't worry about the little pistons around the pack , they are brakes and work off the second pressure pipe.
Hope this helps.