I know nothing about vettes , other than helping a guy bring one from San Francisco to AUS. I have 4 wheel discs on my 67 Coupe. When i pieced the car back together and got to do the brake bleeding , i had no pedal pressure . New master c, new calipers, new lines. I could open fronts and have fluid out, but nothing at rear. I bought a vacuum bleeder, no difference, still no pedal pressure. AFter a week of trouble shooting i tried all suggestions from all forums, no luck.
Last thing i tried was to disconnect the rear brake line at the distribution block and hook up a coupling to connect an air hose to the line. (pressurize the line.) With about 10 psi i Found the problem in a minute. I could not believe it. I could hear air escaping from under the car at the rear. The rubber flex coupling connection to one rear caliper was on backwards(my mistake on reassembly) which meant there was an air gap (skewed seat),between the banjo fitting and caliper fluid in port. Therefore everytime the brake pedal went to the floor it pushed air out, and when the pedal came back up it drew air back into the rear line. (5 min fix)
Pressure bleeding may fix your prob. There was a post about a guy who made a plate to cover the top of the MC with an air line coupling attached to force fluid out each bleed screw. About 1/4 thick with a seal to the top of the MC.
Goodluck. I had a can of gas and a match as last resort.