To prevent soap from gettin into the core of my badger brush I give it a short soak in warm water; shake out most of the water; load the brush with a bit of water on top of the (soaked) soap; build a lather avoiding circular motions as much as possible. I do use circular motions, but very sparingly.

I keep my lather wet and rinse the brush thoroughly after I'm done, ending with carefully rinsing the knot up straight with, again, warm water.

Once a month I do a five minute cleaning with washing up liquid, it has kept my only badger (and main) brush in tip top shape for about five years now.

Best regards,

Pieter