I've read that folks wash their Pt/Pd brush first in tap water followed by distilled water while others use a first soak in Hypo Clear followed by distilled water.
I can't see the point in soaking in hypo or other stuff. That just adds chemicals which you then need to wash out of the bristles because you don't want them in your next coating.
I rinse my brush in distilled water after every coating to keep it as clean as possible. When I've finished I wash it in filtered tap water and finally rinse it once more in distilled water.