I agree that's it's probably not worth the trouble to do it, but if you're set on it, I had this idea:



I know that MEK will swell/soften/dissolve rubber and plastic materials. I suspect plenty of different solvents will work the same. If you have any of that, throw a valve stem or two into a container of whatever you've got and see if it makes it easier to pull the rubber off of the brass. If it works good, you can do big bucket loads at a time. I used to work in a machine shop/plastic factory, and we used buckets of MEK to clean tools and stuff all the time, and when the MEK started getting dirty, we would just pour it through a filter and that would "clean" it enough to keep using for quite a while. Don't let it sit on your skin, though. NOT GOOD FOR YOU.