SMF

I just use a clarifying shampoo and condition the bristles afterwards with a drop or two of coconut oil. I wouldn't use anything harsh on a good brush.

Many shave soaps don't rinse clean and that's why you get build up in the first place.

The only thing I use that's remotely harsh is Barbacide, and I only use that on brushes I sell on eBay or ones I buy used- or horse hairs from China.