There are two allen head drains on back of shock. Remove them and try again. If you have good rebound, ie can't pull them out, that can be good (rebound is working) or bad(they are stuck). If I recall, they have a metal ring like a piston ring.
Wondering if a small amount of heat or boiling them would help. (apparently an oring in there, so be ready to find one)

this place used to have the seals.  not sure if he is still in business as link not working: 
www.nthrepro.com/

Here is a pic to help you visualize the parts when I did mine. Been too long, haha, I can't remember.
yamaha-enduros.com/index.php/forums/rest...tion?start=80#101123