I've had to replace my Clutch Master Cylinder twice in the Z/28 now. I have also changed the fluid a lot just because I track it. Sometimes after a bleed, I get the same effect as you and have that soft spot that doesn't seem to want to go away. So I will get under and bleed it again and sometimes it helps and sometimes it doesn't. The best thing I have gotten was a cheap Vacuum pump. Like:
https://www.napaonline.com/en/p/PTTW87030
I have found a cheaper one, but that is just an example. Hook it up into the clutch reservoir, and pump it up to about 15psi or so. Watch the air bubbles just come pouring out and leave it there for about 10 minutes. Once done, works as it should.