I put a dual master cylinder and power booster on my 1962 Fury.
There seemed to be way too much free pedal travel (maybe just the opposite of your problem).
I found that if the master cylinder was unbolted from the booster, there was a pin with a spherical head that was part of the booster and it pushed on the MC.
This pin was adjustable and the brakes would go from dragging to having too much free play. After some trial and error it's good.
There was nothing in the literature I received about this adjustment.
As others have said moving this pin may give the MC the slack it needs to retract.