I feel funny offering you advice since youíre the professional mechanic, not me.
Of all the vettes I have owned 30-35 of them were early C-3ís and it is the only car that I could never get the back brakes to bleed so they worked right by either pump and hold or with a pressure bleeder.
The fronts are not a problem, the only way I have found is gravity on the rears and it is the only car where you can take out the bleeders and fluid will not run out on the rear even if you let set two days.
Fill master, take out the bleeder on the rear wheels and with master full, slowly push peddle till fluid starts to comes out, let master then run down on its own and fill 2-3 times, then as strange as this sounds seal up the right one first and let master run down again, fill and tighten the left one.
This has always been a fail safe way that works for me and the only way I could ever make it right.
I do assume your bleeders are on top?? Just asking.