Try vinegar! Soak the brush in vinegar for an hour or so until you can bend the bristles. Fill a saucepan with vinegar until the brush bristles are covered. Bring the vinegar to a boil and let it simmer for a couple of minutes. Remove the pan from the heat and let it cool for a minute before removing the brush. Gently comb the brush with your fingers. The paint will still be attached but will fall away as you comb it. Rinse the brush under running water to release the loose paint.