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This is a 47ooo mile very original car.
The front drum brakes will not lock up, they work but not effectively enough. In a normal stop on dry pavement the rear brakes LOCK UP 1st.

Everything has been checked. The shoes are new and so is the hardware and brake hoses. The brake pedal has excellent height.

I think this is a prop valve problem but my understanding is that a 4 drum car only has a brake waring light valve but it does no proportioning. Am I wrong?

Anyone know what size the wheel cylinders should be? Front 9 inch drums. Manual brakes not power.

You shouldn't have a proportioning valve. Just a shuttle valve in the distribution block to trip the warning light. It's designed to move to the low pressure side (front/rear) in case of a leak. It's normally self-centering as long as everything is working properly. It could be that it's stuck partially blocking the front line port, thus cutting the pressure.

Are the drums new also? Or have they been turned? What shoes are they? I have had this same problem with my last two cars and both were traced to shoe/drum/pad issues.

I had exactly the same issue with my '70 Mustang (disc front brakes, but using this to make a point). No matter what I did, I couldn't get that car to lock the fronts. EVERYTHING was new. I even got very expensive cold stopper pads from Ca. In the end I went to AutoZone and got a set of $18 pads. Bang! Fixed. It may be as simple as brake shoe compound or having the drums turned.

You can also use a brake pressure gauge to check the pressure going to the wheel cylinders.

I'm showing 1" bore wheel cylinders for the 9" drums.