Be sure there is no "gunk" build up in the master cylinder or brake lines.
If your "push rod" from the booster to the Master cylinder is too short it will cause excessive brake pedal travel. Too long, might cause the Push rod to go into master cylinder too far and not "release" there by not let the brake pedal return to normal position.
This video will explain what I am speaking about. Make small adjustment, in 1/2 to 1 turn,,,,, small amount of travel here makes a big difference. This is just a suggestion and may may not be the cause of your problem.
Hope this helps,