Immerse the hard paint brush overnight in linseed oil. This helps soften the brush hair for easy clean up later.
Make a solution using one cup fabric softener or hair conditioner and one gallon of warm water. Dip the paint brush in the mixture for ten minutes. Spin it in the mixture until it becomes soft and the paint starts to come off. Wash with hot water.
Warm up a cup of vinegar just below its boiling point. Place the paint brush inside and let it stand for 5 to 10 minutes. Remove the brush when the paint seems loose.
Dip the paint brush in turpentine oil and let it stand for 15 to 20 minutes. Rub the brush against a clean piece of cloth to test if the paint is coming off. Re-immerse if need be.
Use a commercial paint cleaner if none of the methods seem to help soften your hard paint brush.