My vette master is '78, running disc/disc pretty well. When I did mine I had the same problem, now that I think of it. No matter how many times I bled the system the pedal was soft. Turns out it was the speed bleeders. 2 out of 4 were junk, there was some metel shavings/debris from the new lines I did not get out jaming them up. I don't know what you have but it might be worth checking?
