Priming a brush is very important, especially for sable brushes.
Dry wipe the brush on a clean paper towel (dye free) and spread the bristles wide. You will see the white flaky arabic gum which has been used to hold the bristles in shape. You need to dust as much as you can out, splay the bristles gently but move them a lot and change paper towels a few times till it stops dusting.
Then you need to use monomer on it.
I don't recommend anything else, as acetone or ISO can break down the glue inside the ferrule and cause your brush to lose bristles, or become contaminated. Water stays absorbed into the sable too long, which can contaminate your monomer also and stop your acrylic setting.
Use monomer to clean the brush of all traces of arabic gum. If you leave any behind, you will get yellowing of your monomer and you could also cause your bristles to set with the wet gum and monomer combo creating a rock solid glue.
I go through a good half to a full dappen dish of monomer to clean a new brush. And I go through a fair few paper towels. I toss any left over monomer too as the gum traces can cause havoc.
I only ever clean an acrylic brush with monomer. And once clean, I reshape it to my desired shape and don't cap it until it's dry!
Store it flat or bristles down.
This stops monomer seeping into the ferrule and slowly dissolving the glue and contaminating the brush permanently.
Do this with all new sable brushes before their first use, and remember:
Only ever clean your sable brush with monomer.
Even if you get product stuck in it. Soak it in monomer no deeper than the ferrule.