From what I understand, the power brakes are spec'd to have 16 inHg. They still work down into the 12-13 inHg range, but that's pushing it.
I'd just lose the power brakes. I have manuals on my Duster, and I prefer them to the powers on my Challenger. Stopping is the same, the manuals take a little more effort but offer a much better feel. Not to mention the extra room under the hood to get at those rear spark plugs.