I've had this problem.
Make sure you have removed all the lugs and screws and such. There aren't any hidden ones, but make sure you got all the hardware out.
The ideal set up is a rubber foot bench vice, a wooden dowel rod, and a 3 lb mallet. The solution, given those tools, seems obvious, no?
Another thing that might help is to use a thin paintball gun oil to try to use capillary action to lub the orings on the front of the sleeve. Those suckers are tight, so this will not be easy- but just wait til you try putting it back together without damaging those o-rings!
Best of luck.