First I'd try metho (methylated spirits - alcohol to others) ad if that didn't work I's buy some PBR rubber grease used in brake cylinder and master cylinder assembly.
I do remember having a dogged fight with them (at the time I didn't know about metho and i didn't have rubber grease) until I thought to use compressed air - that worked a treat, use a fine "dust gun" inserted under the edge of the rubber and let fly, the air will both expand the rubber and "lubricate" it whilst you ram it home.