455 Rebuild-Now Have Poor Brakes

I finished my rebuild several weeks ago but have only put about 20 miles on the car due to lousy weather. My brakes worked fine before the rebuild, but
now are lacking, especially at a low RPM. While cruising, they seem mediocre, but when coming to a stop I almost have no brakes. I have bled the brake system front to back, and have checked for vacuum leaks. Is it possible my cam is giving me too low a manifold vacuum at idle for my brakes to be effective? (My cam is a Comp Cams XE268H, with 477/.480 lift and 224/230 duration at .050 lift. Lobe separation is 110 degrees. Idle speed appx. 800 RPM) If so, what can I do to fix the situation? Do they make a brake booster that works better with lower vacuum? I will check for leaks again when the weather is more cooperative. Thanks for any ideas.
HD 1974 Trans Am, 455, TH400
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Quoting the movie: National Lampoon's Christmas Vacation.... "Bingo". Try to find a vacuum accumulator kit. Atleast you have room under the hood to put one...

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