"No feedback brakes" go from nothing to full lock

Check the "I'm an idiot" stuff...

• Is the pedal physically making contact with the pushrod.
• Does the new MC work right? Lots of new parts store stuff is pretty hit or miss these days.
• Does it change if you pump it?
• Are any lines (hard and soft) kinked?

As 72bluNblu mentioned, is it happening on all brakes. One thing that really helped me recently was getting a little IR thermometer and checking brake temperature after a drive. I happened to have bad alignment pulling me left and a bad wheel cylinder pulling me right and they cancelled each other out so the only symptom was rapid brake fade since I was stopping the whole car with just one corner.

Ironracer also brings up a good point - vacuum brakes could be the culprit. The wrong size system and you will be waay overboosted.