Power Brake Problems at Low RPM

[mosel70]

My problem is I have power brakes well above idle but no power brakes at idle or slightly above.
I installed CPP's disc brake conversion kit (6370SWBK-6D), all new front brake lines, and put in Summit's disc/drum master cylinder with booster (SUM-760197) and CPP's proportioning valve (PVKS-2).
I'm running 396 engine with Edelbrock 600 (1406) carb. My vacuum line runs direct from manifold port on back of carb to brake booster, a dedicated line. I checked all vacuum lines for leaks with carb cleaner and no RPM increase, no leaks. I checked manifold pressure and found 8 inches at idle and 17-20 inches at higher RPM (say above 2000). So after reading that many on the forum said a vacuum canister might help, I installed Summit's vacuum reservoir (SUM-G1464B) and put a vacuum gauge on the canister. I now have 17-20 inches at idle and 25-30 inches at higher RPM. But, I still lose power brakes after one, at most two pumps once rolling at low RPM.
I'm stumped. Anyone out there have a great idea to bail me out of this??