Brakes

My opinion of the KH 4-pots and 10 x 2 rear brakes, is very high. If the C-valve is plumbed correctly, you should easily be able to lock the front brakes at 20 or even at 30 mph. I've never tried it on purpose, but I think mine would lock even at 40mph.
As to the C-valve the frontmost reservoir goes to the rear brakes. Therefore the reservoir closest to the firewall is for the Front brakes. Those lines are size-coded from the factory, so that they cannot be reversed. The line that feeds the front brakes, goes to the Top of the valve, then splits into two directions, to feed the front calipers. The line to the rear brakes enter the valve near the bottom, then it comes out of the Proportioning valve and continues to the rear.
Between the two lines, inside the housing, is a shuttle valve that responds to a pressure loss on either side of the system, by shuttling to one side, which turns on the dash-mounted brake warning light.
Is your warning lamp on?
Here's where it get interesting;
That light is also turned on whenever the Park-brake is set. So then, if your dash-light is on, you HAVE to figure out why, and if it's not the Parkbrake, then you gotta pull the connector off the Safety Switch. If that turns the light off, then the shuttle valve is grounded.
When the shuttle valve turns on the light, it simultaneously shuts off fluid to the supposedly leaking side. But the valve does not know if the system is leaking, it only knows about a pressure imbalance.
Thus it is easy to suppose that Nothing is leaking, and, as in your case, maybe the front brakes have been shut off, leaving you with just the pitiful rear brakes working.
That shuttle, is supposed to be self-centering, but if it's 55 years old, it might not be happy about moving. So, to get it to move, if the light is on, you are gonna have to create a pressure imbalance the other way;
like
crack the rear line and push hard on the pedal until the light goes out. Then close the crack. Roadtest!
But
if your brake warning lamp is NOT on, and it works, and it is connected, then it is reasonably safe to assume that the shuttle valve is NOT your problem.

Part-2
To check your booster do this;
With the engine off, pump the brakes several times. Then press it the pedal with modest force; keep the pedal down and start the engine. As soon as the engine starts, the pedal should fall towards the floor as the vacuum chamber gets evacuated. say about an inch.
Now,
if, at this time, your pedal is hard, then chances are very good that the hydraulic portion of your brakes is fine. release the pedal.
After the pedal has returned, step on it again. If it drops only about an inch, and then gets hard, I would NOT expect a hydraulic problem.

Finally, with the engine still idling, pump the pedal rapidly about four times.
If the pedal comes up higher, that is normal. Now release the pedal and wait a few seconds, then step on it one more time. If the pedal has returned to it's former height, then the pushrod length is, at least, not too long.
Thus we have proved that the hydraulics are most likely, working correctly.

Part-3
From here, I would chase down a mechanical issue or faulty friction materials and/or surfaces. If I couldn't find an obvious source, then;

I would jack the car up, remove all the calipers and all the rear stuff, then C-clamp each piston into the bottom of it's caliper and C-clamp each wheel-cylinder into it's bore. At this point you have created a hydraulic lock situation, and the pedal with engine idling, should be high and rock-hard with as good as no movement, other than a little springiness in the booster, and a lil springiness in the soft lines.
This is absolute proof that the hydraulics are working.

At this point, I would have to believe that the problem is mechanical; and the first place I would look is for foreign fluids on the pads the shoes, and/or the friction surfaces.
Next, I would look for the pistons not being frozen in the bores.
Next, to the pads being able to sit flat on the rotors, and parallel to eachother..
Next, that the pads fit properly in/on their holders, without getting up.
And finally, that all the front hardware is useable, and nothing compressible is installed between the pads and their running positions.
Now it's time to reassemble the fronts and move to the back.
At the back, the big deal is that the shoes and the drums having matching rubbing surfaces. Look at the shoes. If there is only a tiny patch that is shiny, set the shoe into the drum, on the wear surface, and make sure that firstly it is the right width for it, that it does not sit on an unworn area, and that it does NOT rock in the drum. If it does; The drums and the shoes will need to be matched, and only a brake shop can do this, and not all shops have the tool.
Once that is done, notice that each side has one shoe that is longer than the other, and it goes in, facing the REAR of the car. Then button it up, then adjust the shoes up tight. Then step on the pedal to center the shoes, and again, adjust it up tight. Finally back off each side adjuster, about the same, until the drums can be rotated, with effort, showing that the shoes are still dragging a lil.. Leave like that for break in.
And finally, have a helper step on the brakes while you watch each caliper doing it's thing, and put a bar between the wheelstuds and make sure the system is well able to clamp the rotor. and then move to the back, and with just one drum off check to see that the WCs are working, on both sides, then reinstall the drums..... and all the wheels; jack it down, torque the wheelnuts, and go for a roadtest.