Brushes are always tools that have to be carefully chosen to match the lee. Just like anything, every case is unique and you need to take time to learn the lee, and what they can tolerate on the pain scale.
Myriads
I agree Myriads. I know that well used the brush, a pen or (my preference) fingernails can be very effective. All of them can also hurt, so the technique is very important.
What I actually see is that sometimes you will discover that the ler is inflicting just pain. In some cases you see that the fingernails are not working and then the leer start brutally brushing the soles to get a reaction (and I mean brutally).