Here's something that you might try. Simple Green! Before it came out into the retail market, we use to get it in five gallon buckets, on the tugs I worked, and used it as general engine room degreaser and solvent. Now, I use it to remove pitch from the sawblades in my woodshop, where it does as good a job as commercial blade cleaners. It has a real plus in that it is biodegradable! I don't think it will bother the rubber, but you might give it a test, first, on something less expensive than your presure washer...
