Are the bristles plastic?
Have you been using a cleaning spray with the brushes?
Brake cleaner?
All the brushes the same size, thickness?
Bristles are made PET plastic, if you spray brake cleaner on a soda bottle, same kind of plastic it is frosted and weakened, it it is soaked in acetone or brake cleaner it softens and eventually melts through...
So, no they won't hold up long to brake cleaner...
Brake fluid can make a good paint stripper if warmed in sun...given how it weakens many paints.
FDA and EPA outlawed Methyl Chroride in paint strippers, has to be removed by next year, so if you want a paint stripper that works that can kill you or cause cancer and you do not live in California, then you better be stocking up on your favorite paint strippers that have Methyl Chloride or buying any MEK you can get your hands on. If you live in California, don't even look for it as you likely won't find it already...