On my 72 chevy truck, the proportioning valve must be locked out, in order to bleed the brakes. Check with the manufacturer of yours, to see if there is a procedure to bleed the brakes with their product installed.
Have you changed out all of the rubber brake lines, to include the rear as well?
Last thing to check is that the piston in the master cylinder is wrong, or that the push rod is out of adjustment. The push rod will travel in before it starts to push in on the master cylinder piston. It will give the feel of a soft brake pedal. Seen it happen once, and it was baffling for days before it was figured out. A deep holed piston was installed, when a shallow hole piston was needed.
Google brake booster push rod adjustment to see how to get the correct measurement on the adjustable push rod.