I bought a little of The Masters brush soap some years ago to test it out and I would strongly recommend anyone interested to compare it with some normal soaps in a long-term comparative test (in fact also compare with dishwashing liquid and shampoo used directly on the bristles). I recently got the chance to test the pH of The Masters and it's nothing special - the same as most cheap soaps I've tried (!) i.e. a lot higher than it should be. So I'm never going to use it on my sable and Kolinsky brushes again. FWIW it works fine with synthetics from what I've seen.
This was formulated primarily for caring for bristle brushes for oil painting so it's not surprising that it's not ideally suited to finer brushes. Also, anyone else find little hard nodules in the cake of soap? If so, know what they are?
Bruno, with any brush you use for acrylics remember: prevention is better than cure. If you rinse your brushes properly (and often) during use as I've recommended previously you won't generally get a buildup of dried paint near the ferrule so you don't really have to worry about fixing the problem. Also, never load acrylic paint into a dry brush; this really does help.
One of the best ways of removing any dried acrylic from near the ferrule if you do get it or already have it is with alcohol (acetone if you really have to but it's much stronger). Ammonia is supposed to work well with some acrylic and vinyl paints but I've never had the chance to try it. Auto screen wash and household window cleaners can work too as they often contain methanol and ammonia respectively. All of these will, just like soap, dry the hairs out on a natural brush so it's a good idea to condition it afterwards.
Einion