soft brake pedal
i do have a small master and large volume calipers but the pedal should not fall.
what master did you end up using 1" or 1 1/32" ?
i have the 2 3/4 piston calipers in my duster with a 1" master and it is like power brakes.
..this system is the 4 piston Kelsey hayes which are larger volume and i am using 15/16'
Not sure what you mean by falling, but I'll infer you have that sensation because the smaller bore requires more travel and pedal effort before it starts to grab. That's the tradeoff for more pressure at the pistons. Not sure if that is accurate for your situation.
I have SSBC aluminum KH replacement calipers on my car. The pistons are a tiny 45mm. I now have a 15/16 MC which seems to work. Initially I had a 1 1/8 master, obviously way to big but I didn't realize the effect bore size had on line pressure until I had no brakes. I could push the pedal to the floor and the car wouldn't stop. It was confusing too because the pedal was firm in the garage but apparently was not generating enough pressure at the pistons to stop the car from a slow roll. That's really what it felt like too if that makes sense.
I bled the brakes on my car over and over with no change so I figured there had to be a mismatch somewhere. Both MCs I have are brand new so the chances of both being bad out of the box were pretty slim. Started reading up on the subject a little deeper which led me to the bore size idea. The KH pistons are on the small size so you need the smaller bore master to generate adequate pressure. The '73-up calipers are single piston but actually have a larger area than the KH calipers so you can get away with a 1 1/32 or 1 1/16. 1" would probably be too small for the larger piston, the pedal would travel too far before it started working. Might be your issue right there but don't quote me on that.
Also, the disc brake metering valve is what you need if you don't have one. A drum brake valve is just a distribution block, the pressure is the same on either side. The disc brake valve keeps the pressure to the rears lower when you press the pedal so they don't lock up before the fronts start working. Incidentally, if you have a disc brake valve and then installed another prop valve after that, its going to decrease the pressure to the rears even further, even if the valve is backed off all the way.
Not sure if any of this is redundant or relevant to you but just thought I'd throw it out there.