Once they start breaking down, I think it's pretty much close to impossible to "rejuvenate" them as they are organic materials that break down internally. You might be able to coax back some moisture on to the surface of the hoods, but once they darken, crack deep inside and tear at the edges, it's pretty much a losing battle. Our only hope will be more people deciding to manufacture reproductions with hopefully better materials that will make them last much longer (silicone?).
Chombi