Braking Problem. Do I need a Vacuum Canister?

[AusTx68]

Since adding rear disc brakes my braking has suffered. When cruising down the road, the first time I hit the brakes they all grab really well. Each push of the pedal immediately after results in harder pedal with less braking action. If I need to quickly slow down after the initial pedal I have to really get on it. It can be a little scary at times. I'm guessing it's a vacuum issue but I'm not sure. Here is my setup:

New Stock 11" Vacuum Booster
71/72 Front Disc Brakes
Captainfab Rear Brake Setup (Camaro calipers)
CPP Master Cylinder MCPV-1 (has built-in prop valve)

The master cylinder is adjustable. I have pressure to the rear brakes set to max.
My cam duration 245/245 @ .050" lift