After the cracking and weathering has taken place, there is little that can be done to restore the parts. If the part is one where strength is not a factor, you can take one of the black leather glues, or rubber cements on the market and rub into the rubber, filling the cracks. Most good hardware or auto stores have black rubber cement. If the rubber has hardened, but is not cracked, it can often be softened and made pliable again by rubbing glycerin into the rubber, massaging well. Make sure you don’t stretch or break the rubber. Some restorers have used a good quality, heavy duty brake fluid, which is compounded to live with synthetic rubber in place of glycerin. On parts such as brake hoses, where strength is vital, if the hoses are badly cracked or weathered, there is little that any restoration method can do to help. The brake fluid treatment would help if used before cracking starts. Good Luck!

I once had a rubber bushing fall into a small cup of oil and it sat in their for a good week or so. When I pulled it out it was pretty soft, I never let it sit out of the oil so I don't know if it would dry hard again or remain jelly like?