‘66 Barracuda Brakes Suck — Proportioning Valve Maybe?

I wish I had an answer for you. I just put KH discs on the front of my Duster and I'm not impressed with the improvement over 9" drums. If someone installed the discs on the front while I was away and I came back and drove the car, I wouldn't even be able to tell there was a conversion done. Although mine has a soft pedal until about halfway down and then it starts to get firm and feel normal. Rear brakes are as tight as I can comfortably get them too.

Had a 67 Valiant years ago with drums all the way around with a rock hard pedal and only about 60% of the braking power my Duster had with the same size drums. I changed out every single thing other than the hard lines on that car and I could never figure it out, even with all the advice I got on here.

Your prop valve looks to be in bad shape but you said that the brakes were normal back with the old single pot MC. I know it sucks bleeding brakes but I'd swap the old MC back on and see if the brakes feel normal. At that point at least you'll know it's the MC and nothing else that's causing the problem.