I had the same concern a few months ago regarding some rubber on the bus. I needed to clean the rubber first and get all the 20 years of grime off. I know it sounds funny but I put the old rubber in a five gallon bucket, and then put on quart of the oxy clean poweder like stuff from the dollar store. It foamed up big times and I let it sit for several hours. I then rinsed off the rubber and did it one more time for another day. As a natural born cynic I thought no way would this work. Much to my suprise the rubber was not only clean of all the grime and build up over twenty years it was also rejuvinated. It worked great!
I spent five bucks and rejuvinated the old rubber seals. Try it. The local rubber hose supply house suggested a product from 3m that was $35 a pint and I think my stuff was cleaner and looked better. They also suggested an industrial quality citrus cleaner?