I’m sorry I didn’t get back with you last Friday.
What are the brushes used for? If they are just used for dusting, and don’t have any dried paint or other residue on them, I would suggest using a mild surfactant such as Orvus Paste (http://www.conservationsupportsystems.com/product/show/orvus-wa-paste/detergents-soaps).
Just be sure to rinse out all of the surfactant well after washing and you will only need to use a tiny bit to break the surface tension in the water.
Also, brushing out the bristles with another, stiffer bristle brush will help to remove any loose debris.
Let me know if this helps!