I installed Sumitomo MK63 Calipers and vented rotors on my 72 Z recently. I have experienced a hard brake pedal since that installation. I tested the vacuum assist and I believe it is working. I pumped the brakes, held pressure and started the car and the pedal depressed slightly. I plan to put a vacuum gauge on it tomorrow and see if I have at least 18" vacuum. I believe the next possibility is the brake valve (Pressure Differential Switch). It may have moved off center during the installation