Chlorinated brake cleaners (such as part #1420) are made up of solvents that contain chlorine as part of the molecule. These chemicals include perchloroethylene (aka. tetrachloroethylene) and methylene chloride and are generally non-flammable, extremely fast-drying, and provide the best cleaning for brake parts. The non-chlorinated versions (such as part #2420 and part #2420c) make use of solvents without chlorine and are fast-drying, have a more favorable toxicological profile, and are flammable. Some states and municipalities have banned of the use of some chlorinated solvents, so these non-chlorinated products were for quite some time the only option. However, we also offer a hybrid product (part #2520) based on a permissible chlorinated solvent now, too.