72 K-10 Brake System Problems

[azsmokey]

I would appreciate any ideas, feedback or suggestions on this matter. A little about the build and setup before I get into the specifics.

The truck is a 1972 Chevy K-10.
-2003 Silverado SS drivetrain
-6.0L LQ9 motor with Vortech supercharger
-4L65E trans
-NP205 T-case
-4 wheel disc brakes
-All new stainless brake lines
-8inch Dual Diaphragm Booster/Master/Combination valve setup from Right Stuff detailing via Summit http://www.summitracing.com/parts/RSD-J85315672/
-

Our problem is with brake pressure. After setup and brake bleeding, there was barely enough brake to hold the vehicle in place while idling in drive. Initially, we did not bench bleed the master cylinder. I completed that yesterday, re-bled the system and the problem persists. I checked the vacuum and we are reading between 17 and 20 vacuum, both via the dash vac/boost gauge and a separate vacuum gauge. What am I missing?