Poor brakes at low RPM

[John]

I am new to this forum. I just purchased a 71 Chevelle, I am very disappointed with the braking. I've noticed the power brakes are very bad when at idle and low speeds. This is my 2nd 71 Chevelle and never had issue's with power brakes, although this one is not as stock as my previous. It's got a 400 bored out to a 406 with a pretty beefy cam. Any suggestions on what to look for first? Any possibility it's the booster?

Any suggestions would be greatly appreciated.

john