BCP:

Put the brush in a bottle of acetone and forget it for a week.

Best if you have a bottle you can close, make a close fitting hole in the lid for the handle, and hang the brush with the bristles about 1/4" from the bottom. Seal around the handle.

This will also recover a brush with epoxy hardened in it.

Be advised though, some brushes have the bristles glued in. Guess what happens to the glue?

Bill.