Licensed!!!!!!!

Got the Z28 licensed and insured and legal as can be. One trip around the block and had a brake line pop. Easy fix. I then took it on a 10 mile shake down run. Everything seems right except for another brake problem. The car will stop, but thats about it. Brakes seem really weak. Pedal is soft. Will not lock the wheels up no matter how hard you hit it. Adjusted the rear drums, bled all four wheels, checked the booster and the check valve. Seemed okay? What do you guys think? Proportioning valve? Master Cylinder leaking back on it's self? It hasn't been on the street since '02. Any suggestions or experiences welcome! Thanks, Dave

The master cylinder could be internally bypassing. Hold the pedal down and barely let up on the pressure and see if the pedal continues slowly to the floor.