One of the reasons why it can be hard to remove water wash-up coatings from a paint brush is because the wet coating gets drawn high up into the "heel" of the brush by capillary pressure. The stuff that gets drawn up into that area where the bristles are tightly packed together will dry there while you're painting and result in "shaggy dog" paint brushes where the bristles are spread out and the heel of the brush is hard as a rock.

One way to avoid that problem is to wash the brush out in water and shaking out the excess water BEFORE using it, and to periodically add some water to the bristles near the ferrule using an eye dropper.

Doing that ensures that there's water high up in the bristles which will prevent the water-clean-up coating you're applying from drying up and hardening in that area, thereby ruining the paint brush.

You can buy an eye dropper for less than $2 at any pharmacy.

If your brush is already hard with dried up stuff, I would dip the brush bristles in acetone or lacquer thinner, working the brush bristles after each dipping. Don't just leave the brush bristles soaking in either solvent. I expect both solvents would probably dissolve the Thompson's Water seal but you can check to see which one works better by dipping a Q-tip in either solvent and using that wet Q-tip to clean a small area of that Thompson's water seal. Clean in an area where light reflects off the Thompson water seal. If the solvent dissolves the stuff, the surface of the area you cleaned will be rough and will scatter light diffusely rather than specularly. That is, the cleaned surface will be dull instead of glossy.