Right, it is definitely something with the caliper design. I suspect its a combination of the prop valve and the rear disc ratcheting calipers, the prop valve GM part number in the catalogs is very specific, one part number for 79-81 Firebirds. The one prop valve fits all the aftermarket sells I am sure is not exactly tuned how GM intended.
I will know soon if an adjustable prop valve and/or hydroboost makes the brakes behave better.