How can I fix my 1977 Buick Riviera Brakes. ( 6.6 litre )?

Just overhauled the Brakes on my '77 Riviera. Four wheel disc. New Discs, hoses,Master Cylinder and pads. Pedal feels hard until the engine is started then the pedal almost reaches the floor. The car will stop fine, but if you sit with foot on pedal it seems to give slowly. There are no leaks and the system has been bled correctly. Can the Booster be the problem, with too much boost ?