I have a bunch of brushes with dried paint and glue. I've tried to clean them up but they're still hard. What medium is best to restore my brushes?
You need a Brush Cleaner and Restorer formulated for this task. There are a few of these available, only in art supply stores; you'll want to check with the ones around you to see what you can purchase.
If you are in the United States, Winsor & Newton makes a widely available product I have used with great results; it's formulated for dried acrylic and oil paints, but I have also used it to dissolve away dried latex (house) paint, as well as years-long set dry varnishes. With this product, you'll let the brush soak up the solution, checking periodically to monitor the progress. In a low quality brush, it may end up damaging it by damaging any glues holding the bristles in place in the ferrule, but at the point you're using a restorer on a brush, it's typically ruined anyway, so there's no risk in trying. In my experience, I've only had one or two brushes lost this way out of dozens.
Unfortunately, this particular product is not available outside of the United States, but other companies do make similar products that you'll be able to find by asking your friendly local art supply shop. The employees there should be able to recommend a product to you with which they're familiar.
Ultimately, the best option is to avoid needing a restorer in the first place; take proper care of your brushes, cleaning them right away instead of letting paints dry in them. Avoid getting paint up into the ferrule, and dry them bristles down so paint and water don't soak down into the ferrule and cause damage.
As an alternative, I have used a low-fume paint stripper to smother the brushes in, and then wrap in clingwrap, and let soak for several hours. (I have not tried paint thinnners: this probably works better with oil paint than others. Correct me if I am wrong.)
These may be off topic? Happy to delete.
Restoring big brushes For the really stubborn bits, I've used a heat gun on a low setting (but only on bristle brushes, not plastic filament, as I found it melts/deforms a plastic brush) will soften paint, especially plastic paint.
Then use a painter's comb or even a not-too-hard wire brush to remove paint. You may need to go through the process more than once. I have also boiled brushes in water to soften the paint, and then scraped the paint out while soft.
Cleaning artists oil bushes I use mineral turps, and then pure soap all the way down to the ferrule (push the brush into your soapy palm with warm water). Lay on an old towel. Then to help the brush keep its shape, coat it lightly in petroleum jelly.
I was using SARD wonder soap, but this leaves a residue.