I have of lot of expensive artists brushes that I use for model work. Some of these are over thirty years old. Over the years they've acquired a lot of dried paint build-up at the top of the bristles near the ferrel. The paint in the bristles has rendered many of the brushes useless for fine model work.
My solution to rejuvenate the brushes is to soak the bristles in 91% rubbing alcohol for several days. After the soak I scrub the brushes on paper towels to remove the softened paint and then I wash them in soap and water. The hardened paint is gone and the brushes, although not "new," are soft, pliable and ready to use.
Tip #2 - To prevent the paint from building up in the first place, always wet the bristles in water (or solvent, if you're using solvent-based paints) before sticking them into the paint. The wet brush will keep the paint from drying on the dry bristles in the middle of the brush.
I got this alcohol tip from an artist's magazine.