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

Your tip using boiling water worked but it was not easy. Removed the 2 x plugs in the rear as well. Find that these are Thermal-Flow mx shocks w/ a 12.5x28x12 seal on the shaft. No seal provision in the aluminum top screw cap. The only only O ring I could see after disassembly is in the reservoir cap.

PABDT he no longer makes the parts for the shocks. I was able to track him down and ended up buying a bunch of the seals after he called it quits. They are currently in the process of being reproduced and hopefully we will see them soon.

Installed two new 12x28x6 oil seals stacked together in place of the OEM 12.5x28x12 seal & assembled the Thermal Flow's w/ new shock fluid. They now function OK w/ no leaks.