Excellent post, Ray. The only addition I can offer is a picture showing where all the lines go, and the port sizes:
Ray, you're right about the business-end being on the left (for the rear brakes). Since the front brakes don't get a pressure or flow reduction, those three ports are open to each other.
The rear brakes pressures and flows are controlled on the left side of the combination valve. There is a spool valve in the middle that controls the electrical connection, and it is designed to prevent fluid from pouring out if one of the lines ruptures (or a wheel cylinder leaks). The spool slides over to one side (closing off fluid on the 'bad' side) and it closes an electrical contact, which connects to your warning light.
For simplicity, I use a "12-volt flashing LED". No resistors or other components are required. Just connect one side of the LED to +12, and run the other side to the proportioning valve switch. The other wire on the switch goes to chassis ground.
If anyone needs one of these LEDs, let me know. - Dave