I have started new posts and have reply'd to many about my brake issues for over 2 months now. I was so desperate that I took it in and had them power bleed them today, and they found my problem in :30 seconds. It was so obvious that it never crossed my mind. Mainly because i never really thought about it. I changed to a 77 corvette master cylinder. bought the in-line p-valve, new booster, and all new calipers. The entire system held pressure, with new everything, i bled them with 3 big bottles of brake fluid and still spongy brakes that didn't work unless you pump them 5 to 6 times. My problem went all the way back to the picture below and a sharp eye would be able to see it. A 4 wheel disc brake conversion that had me wishing i never converted. Try taking some geuss's and i'll tell the answer after a bit.