Disc Brake Conversion Problems on my 66 C10

[Bushman66]

The equipment: 77 Spindles, 77 C10 Master Cylinder, 82 Corvette Booster, Captain Fab Booster bracket. Booster and MC have the correct shallow intermediate pushrod. POL Factory Proportioning valve. All new hardlines and rubber lines in front. Rear factory drum brakes rebuilt 2 years ago.

Everything went together fine. Bench bled the master, noticed the small (rear) chamber was harder to bleed than the front. Ended up tilting the MC in the vice to get the rear chamber to bleed fully. Bled the brakes the old school way, noticed several leaks, so repeated with a Motive power bleeder. Bled with the centering tool in the proportioning valve. All wheels are flowing clean fluid, no problems. No more leaks that I can see.

The problems:

1. When applying the brake pedal, it feels a bit spongy for the first third of travel then firms up. When applying the brakes, the engine feels like it idles down a very noticeable amount, its a stock 350 crate motor. 11/32" Vacuum line run to the booster from the rear brake port of the Edelbrock Carb. A booster should not affect engine idle?

2. When applying the brakes, I had the truck in the air, wheels off, and the rear brakes do not stop the rotation of the wheels, even with a FIRM push on the pedal. Rear drums adjust to have a light drag all the way around when turned by hand. Turning the front wheels and then having my fiancé apply the pedal, the fronts seem to be working fine. Rear passenger drum gets very warm.

Shouldn't the rear brakes be able to stop the rotation of the rears, even with the truck in gear? Or is the proportioning valve sending so much bias to the fronts that it wouldn't?

I'm beginning to suspect i got a bad POL proportioning valve. I bought it last year, heard they were being plagued with some bad ones?

Really stumped here, just want this project to be finished...any help greatly appreciated!