Ideally you'd want a bit more vacuum than that, but if you're sure you do have that available at idle, you can use a power brake booster. You're going to want about a 1200 psi capability, so best bet is to use a 9 inch dual diaphragm booster. Some info is here.What master cylinder to use with 11" front GM disc and 11" rear Ford disc? I have arround 17" vac at idle. Do I use power or manual? I have not checked the pedal ratio yet. This is on a street rod and will be firewall mounted. Thanks.