Rear disc brake problem

[Ackattack]

I have LT1 camaro calipers installed on my truck with new everything (rotors, brake lines, disc-disc proportioning valve, MC, and line lock for the front). The brake booster is not new.

I currently have the rearend up on jackstands, when I put the truck in gear, I can barely get the rear tires from turning at idle if I push on the brake pedal with all of my might.

I bench bled the MC, bled the brake lines, fixed leaks, replaced a kinked SS flex line, bled the brakes lines, bled the brake lines some more.

I would think disc brakes should have no problems stopping the rear tires when up on jack stands, I mean if I drove it, they would essentially be doing nothing.

I can't really think of anything else to do except start replacing new parts. [BTW the proportioning valve is from CPP and the MC is a remanufactured one from Autozone for a 72 if it matters, and I rebuilt the calipers]

Anyone else have similar problems, or suggestions