'75 Duster intermittent brake problem

I had a similar issue, turns out the rear brake shoes weren't adjusted tight enough.
Bled the brakes 20+ times, never fixed it. I ended up swapping to discs in the rear instead of replacing all of the adjusting hardware.
What I did to verify this was to clamp the rear rubber line with vice grips ( protected with cardboard ). Hard pedal instantly with the rear line blocked off.
Still could be your front brakes, but if the bleeders are stuck, time for new calipers.

TrailBeast's diagnostic is spot on if the master cyl is on it's way out. Light pressure on the pedal wont flare the plungers to build pressure, but a quick jab on the pedal will.
I also had this happen with a "rebuilt" one from a parts store. Bench bleed didn't feel quite right, thought I was doing something wrong, but it was bypassing internally right out of the box.