For those of you who are considering doing brush restoration, and if you have a drill press or can get your hands on one you may want to cosider the following. Prep the old brush the way others have posted on this webb, but after you get the top part of the brush cleaned up you may want to consider using a Forstner bit. A Forstner bit drills a flat bottomed hole, not like a conventional twist drill. With a flat bottom it make an ideal surface for gluing in a new brush knot. I would recomend cleaning out the top edge of the old hanle and then setting the brush handle up in a vise for stability while drilling. With the top edge of the brush handle cleaned out you will be able to align the bit up properly for the finish cleaning. The bit depth is almost perfect for the required depth to seat a new knot.
I have attached some pictures of a Forstner bit and a drilling set up that I have used in my shop. Note your can purcahse the bits in 1/16 and some times 1/32 incroments. A 15/16 diameter bit is perfect for a 24mm knot.