Rubber can become hard for a number of reasons, including temperature, exposure to the elements, quality of the rubber, age and improper care. To keep rubber in good condition, it is best to keep it out of the sun and harsh conditions, and to treat it with a rubber treatment made for your particular item. There are many commercially available treatments made for the preservation of tires, seals and other rubber items. If you have a rubber item that hardens, there are a few things that can be done to soften it so it can be useful.
Scrub the rubber item with wire brushes and warm water to remove all the dirt and oils.
Fill a container large enough for your items with a mixture of 2 parts water to 1 part ammonia.
Place your items in the water/ammonia mixture and make sure they are completely covered by the water.
Soak the items for one or two hours until they have softened. For larger items, the water may need to be boiled lightly and replaced as it evaporates.
Leave items to dry completely before putting them away.
As an alternative to soaking with ammonia, items can be rubbed with glycerine and heated in warm water.