Since the 1960's when hot rodders wanted to put in performance camshafts, the vacuum needed to operate powerbrakes has been insufficient. They were able to get by with a manual transmission, but with automatics, overdrives, and fuel injection, standard vacuum boosters just will not stop a high performance car. 4-wheel discs require even higher pressures. Street rods are so low that any power brake on the frame will hit the ground. The 7" common street rod booster can only produce 450 psi, with vacuum.