"raymondj"
I clean white china bristle brushes using paint thinner and one of
those metal combs used for cleaning brushes. This appears to work but
after time the amount of "usable" bristles gets smaller. From the
metal piece growning downward it turns into a rock.

I purposely try to get this section by turning the brush upside down
with fresh paint thinner on it and working it with the comb.

What am I doing wrong, or is this just life? Thanks.

A few thing I find helpful:
Clean brush *immediately* after using it, and put in freezer between coats,
rather than wrap it or stick it a thinner can.
Work brush briskly in pint of thinner, bending bristles and sloshing well.
Then hold brush, bristles up, and pour lots of clean thinner slowly down
towards the handle, squeezing dry periodically , freeing paint at the base
of bristles.. Once fairly clean, I dry thinner-soaked brush in large rag,
then hand wash brush in warm water and dish detergent to get the thinner
out, squeeze till just damp, and wrap in paper towel tightly to settle
sproinged hairs, and to set the brush shape. Put in dry place to thoroughly
dry. Leave it in the paper, for next use. I know this is complicated, but I
have good brushes that have been used for 15-20 years and are still usable.