Wintergreen Oil....mix w/water ... about 2 Oz. in a quart of water. Bring to a boil on the stove with the rubber parts to be softened. Turn it off and let it cool. It'll smell up the whole house so get permission first. I've used it on carb boots that were high $$ new, and it made them pliable enough to be useable again.
IBA Member #49541