Bad Check Valve???

Well, I installed another M/C and bled it out etc...Just came back from a little cruise around town. Something's still not right with the Booster/Master. It will gradually drop down further and further with every stop. Then when I shut off engine, the Pedal pops back up. It's like something's applying the brakes even when I'm not stepping on the pedal BUT there's no slowing down. In other words it feels like another bad M/C. These are A-1 Cardone's. It's definitely better than before but still not quite right.