Randy D. has a good point on checking the linkage pivot assembly behind the booster.
An added adjustment that you may want to confirm is the
brake booster rod to the pivot assembly he has mentioned.
There is a connecting bolt/nut there that is eccentric and it had an Index mark on it. It is at it's maximum eccentricity when pointing Rearward. This adjustment assures
that there is free play between the linkage/pedal and the master/booster. You can fell this play by pulling the rod toward the front of the car. If there is no free play, get some by this adjustment or it is possible for the brakes to have residual pressure from the master.