Did you adjust the length of the rod from the booster to master cylinder? Its length is critical. To short and your pedal drops toward the floor and will not return to upper stop. Too long and your brakes will drag. The gap between the ball at the end of the rod and the master cylinder socket is about 1/16", I think. You didn't say, but if you didn't buy the blooster/M.C as a matched pair, the master cylinder sockets are different depths for disc or drum. They sell a "pill" or short rod to make up the difference on the deeper socket type.
