Power Brake Problem

[69sixpackbee]

Heres the scoop:
My '72 has new calipers, rear cylinders, pads...you know, all the stuff. I have a brand new booster an a good master. This problem I am going to describe happened before I swapped the booster out as well. When the engine is cold and fast idling and the truck has been setting for a while, the brakes are nice and "touchy" as in barely any pedal effort to send you through the windshield. After I go a few blocks the brakes need alot more effort to stop. Even when going slow. The engine has 18-20" vacuum when idling warm and has no leaks in the power brake booster because the idle is nice and smoothe all through the pedal travel. This acts like a poor vacuum condition yet the engine has plenty. It is a stock 402. The connection to the vacuum booster is tapped off of the manifold runner behind the carb. This is a Performer dual-plane intake. Should I attach the hose to the carb base instead or what does this sound like?
Thanx!
Bud