Is your master cylinder the old GM style that came on the 67 Chevelle with standard brakes (narrow top) or is it the fatter top master which was used with the later Chevelles with disc brakes? You need the larger one at the very least, because the disc brakes do need a larger volume of fluid.
But I saw this problem once before on an older Chevelle and it wasn't the master cylinder. The one that I had problems with was because the Wilwood calipers had been just bolted on and were not centered and square with the rotors. The caliper pistons retract easily, so that you don't have drag and this was causing the brakes to need to be pumped up to set the pads back against the rotors. You need to make sure that the calipers are centered with the dividing line in the center of the rotor from top to bottom.
Bill M