Brake issues on power brake update.

[ricktfd]

I have a 64 SWB C10 with brake issues. It has a 5 lug conversion, with disc front/ drum rear, but was non power. I recently installed a new dual booster/ master cylinder unit from CPP. I have good vacuum and the check valve is working properly. When I brake for regular stop and go traffic I have decent stopping. But at highway speeds or on hard braking, I have to stand on the pedal and hope I can get it to stop. It is as if I didn't have power brakes at all.
I am at loss on where to go now. Any ideas would be appreciated.