Based on Bobbi Brown's makeup book, you can use diluted baby sha

She said the brush cleanser solution is just mild shampoo, like baby shampoo. I bought Johnson & Johnson's baby shampoo, and dilute it to clean the brushes.
The foundation brush is a little bit hard to clean. I use baby oil first. You can use a small container, dip the brush into baby oil, and gently make circles to disolve all the foundation. Then you can use the diluted shampoo to wash off all the baby oil.