I like multi-tasking and saving time. When I bathe the horse and am finished with the soapy water in the bucket, I throw in all the brushes. Let them soak, while I braid or band & put his jammies on. Then it's time for me to clean up my washrack area, I dump them out of the bucket & hose them off in the washrack to clean up the whole works. Been doing this for decades.
I use either a mild soap (like Ivory) or horse shampoo. Pour it in a big bucket w/ soapy water, and let all the brushes soak for about 15 minutes. You want to make sure not to use any harsh chemicals in case any residue is left on the brushes - I wouldn't risk getting bleach on my horse's coat!