68 Dart GTS brake problems

Sounds like the booster is not working. The pushrod may need adjusting.
Try this; With the car turned off, pump the brakes about 8 times to rid the reservoir of trapped vacuum. Then push the pedal about 50% down and start the engine. Almost immediately as the engine evacuates the booster, the pedal will fall down about 1 inch with no help from you. If yours does this, the booster should be fine. If it does not perform like this, then,the pushrod may require adjustment.

When you replaced the flex-lines, are they original rubber-type?
A high,hard pedal, that releases, and doesnt hit the floor, is usually a good thing.