In any event, I've seen it happen.

Very interesting. Although I can't argue with a consensus like the one in this thread, I've never seen anything like that blamed on a master.

If one circuit dies, the brake failure light comes on (if not rusted in place I suppose) and the pedal drops an inch. Braking pedal force is doubled, and it pulls a bit when one rear brake becomes totally useless. But, second hand symptom reporting is not the same as having driven it yourself.

Also, every master I've replaced was easily fingered by the characteristic slow bypass or sinking pedal when held firmly. They don't magically recover, in my experience.

It can't hurt to renew the master, but I would be very skeptical of calling it "a fix" once done, without eliminating the even simpler explanation at the caliper(s).
--
Art Benstein near Baltimore

Always remember that you're unique. Just like everyone else.