I've had rubber hoses that I have "saved" using heat-proof grease. Works wonders. Not sure it works on crumbling rubber but I've seen it work on cracking rubber. $2 or $3 dollers in the plumbing department.
Radiator anti-freeze Coolant, it has the best conditioners for rubber in it. If not your hoses in the engine compartment in the summer heat would crack like crazy. Wipe on old tires if drying out, makes it look good again. I would soak the rubber in a container of the anti freeze. Not sure what you are doing it to. I have hard dry rubber gaskets on bottles. I dont really mess with it. It does work though.
Watch that crumbling one. It might not help when they are that far gone. Please let me know how it comes out. I am kind of interested. Thanks.