Not everyone is a chemist, I'm not either, but some of the comments here need to have some practical knowledge applied, and as with some others, I am amazed that their boots didn't end up in the next county, along with their pot they were soaking in.
First, solvents are petroleum by products, rubber is a petroleum product, hence the brake fluid expansion and the Xylene/Acetone reactions. by using these products you are altering the base ingredient, rubber, but you are not cleaning it, nor are you removing the gasoline that has become part of the inner surface. I am not saying it does not give some kind of results that soften the boots, probably does, Dangerously.
A cheap stainless steel pot from the dollar store, a piece of folded up hardware cloth in the bottom, dump in your boots, fill with water above the boots, take a guess at gallons but don't fill the pot ( no reason to), dump in 4 oz of wintergreen oil. turn on the heater beneath ( I use an old turkey burner but you could use any type of heater, BBQ grill, whatever. There is nothing flammable in this method..
I heat to a low simmer/boil, nothing violent, and leave them for 45 minutes. then out they come and into a bucket of clear water, rinse/cool, and wipe down with a cloth. You will see most of the crud on the surface of the water,which is usually foamy with oily crud on top. If the boots are still dirty after this, it will be residual dirt on the surface, I have found that a vinyl/rubber cleaner from just about any source will clean the surface dirt off. When I am done I have soft boots that make mounting to the head and carbs alot easier, with no danger of explosions. and they stay that stay for a very long time, and they do not smell for long either.
This is what works for me.
