How do I restore paintbrushes ruined by dried latex paint?
I use a product by "Clean Strip" called roller and brush cleaner. It usually works well even on badly hardened paints.  Word of caution however, try to use it outdoors as it really has a lot of very offensive fumes. Also wear plastic gloves as this stuff will burn your hands. Heed all the warnings on the can, this is nasty stuff, but works well.