I have had reasonable luck soaking brushes in straight Murhy’s Oil Soap to get even stubborn paint out. I clip a clothes pin to the handle so the brush is not resting on the hairs directly. When drying them I hang them from clothes pins and let them drip dry
Once paint gets worked up under the ferule it can be very difficult to get all of it out, I try to avoid getting it there in the first place, but notice I said “try”.