"No feedback brakes" go from nothing to full lock

Here's my take.
Your pushrod is too short.
You step on the pedal and nothing happens, so you push harder and harder until finally you are tickling the M/C. But now you are pushing so hard that when the feedback hits the boosters control valve, BOOM! you got boost!
The Solution is, in my opinion, simple; just increase the pushrod length, little by little until you get the feel that you like. Try an initial increase of three turns, but easy on the pedal on the first test-drive.

Be advised that it is possible to make the pushrod so long, that a panic stop could break the control valve. If that happens, the brakes at the wheels will be activated nearly the instant you step on the pedal, and no amount of pushrod adjustment can compensate for that.