Given time boar upside down will burst and handle. That said:
Find a model RC car or airplane shop near to you. Or sometimes craft stores have it
another source is Woodcraft
Buy Satellite City Hot Stuff Instant CA Glue, Thin, 2 Ounces at Woodcraft.com
Ask if they have "Hot Stuff "Thin" Cyanoacrylate Glue"
Flush the crack with denatured alcohol and let it dry.
Read the instructions on the bottle will wick into the crack and you clamp it tightly.
The advantage to a walk in shop is you can take the handle with you on a Saturday or Sunday and the guys will know better what to do!
Good luck, that brush is worth saving.
~Richard