Members with 4 wheel disc systems: Can you lock your wheels?

Kern Dog said
I can't. I rarely ever have.
Well, I have locked one or two on wet pavement and the dirt surface in front of my shop. On the road though? Only a few times

I am having a hard time making sense of my brakes. I've tried multiple combinations and no matter what I do or try, I can't get them to skid on dry pavement.
I'm not a trained mechanic but I do have years of experience with these machines. One method that I use is the process of elimination. If the system has faults and you change enough parts, eventually you will find the problem. This is not easy on the wallet but sometimes it is the only course of action you have to work with.

This car stops well but it isn't awe inspiring. I never feel like I need to lift from the brake pedal because I am about to skid or stopping faster than I want. I have good parts in the car but something just isn't right.
Originally, it was a 4 wheel 10" drum system. You all know how those perform.
My first change was an A body power booster and 11" front discs. It stopped well and never had me feeling like I was scared to drive it fast.
Later, I upsized to the Cordoba 11.75" front rotors.
In 2006 I pulled the rear drums and installed the Dr Diff 11.7" rear disc brakes. From this point forward, the car got faster but the braking stayed the same.
Last year when I had the engine out, I decided to upgrade the brakes. I installed the Dr Diff 13" front kit and went down the rabbit hole in a failed Hydroboost swap. That resulted in a complete failure either due to a faulty HB unit or my impatience in the bleeding procedure. Regardless, the HB came back out and I tried a manual 1 1/8" master cylinder. TERRIBLE. Hard pedal with horrible stopping force. I tried a 15/16" MC. Better, but still not nearly good enough to be content with it.
I put the A body booster back in along with a vacuum pump and tank and that is where I am today.
It stops good enough but I'm still wondering why it won't skid. To me, it seems like if it can't skid, then the system isn't at it's potential.
I bought a brake caliper gauge....

I only measured the rear and it registered 1200 psi.
I'm open to any and all suggestions except swapping drums back on the rear. There is something wrong and I am determined to figure it out.

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Without ABS you should be able to lock all four wheels on dry pavement in a "panic" stop. It should be difficult to lock the rear brakes. You do not want the rears to lock first as the back then wants to pass the front.
Brake pressure is a finction of the pedal ratio and booster multiplication. Frequently the balance or bias valve is to reduce rear pressure preventing rear lockup. Drum diameter and widyh or rotor diameter and pad diameter work in conjunction with piston bore diameters to create brake torque or force. The master cylinder diameter converts the pedal force to line pressure. Wheel cylinder area converts the pressure to pounds force.
Standard accepted on our old cars is that under braking with weight transfer, the fronts do 70% of the breaking, on average. The front wheel drive cars shift that to 80% to 95%. Rear engine cars with more weight at the rear shift this closer to 50% to 60%.
In your case you should be able to lock the fronts but not quite the rears. Modern tires have considerably more grip than the skinny bias plys when these cars were new, which combined with wider tires creates more force on the brake surface to lock them.
You need to compare front and rear line pressures out of the differential valve and multiply by the piston area of the caliper. This will give the force applied to the drum or disc. Once. You know the line pressures I would talk to the component supplier for advice.