Robert, we grabbed some practically-useless brushes which hadn't been properly cleaned to spread paint remover on a dresser we were refinishing. Both cheap-o plastic bristle and expensive natural bristle brushes came out like new, regardless of the nature of the paint that had left them stiff or their centers gummed up. I didn't even need to comb out the bristles during clean-up.

Paint thinner would also work, but it'll take longer because it's weaker.