67 400 need some help with vaccumm pressure

[Harley91MR2]

i have a 67 bird with a 400 that was overbored 30 to 406, had it built at california street machines in suisun city, ca, and it has a 500 lift cam in it that has been giving me trouble with vaccumm pressure.., you have to rev it up to get any kind of pressure to the brakes.. so sometimes i have to shift it into neutral to rev it up to stop, also i have a holly 750 with vaccumm secondaries.. i wanted to save a little bit of gas, i should have gotten the mechanical secondaries though.., but anyways, it takes a while for the secondaries to open up.. when they do it will really go, but as it is now i beleive the low vaccumm pressure is making the secondaries open up way late.., i've heard of vaccumm reservoirs and other stuff.. but cant seem to find anything specifically for the firebird, can anyone give me any help or advice to fix this problem?