Wire going to prop valve

[RichardJ]

Actually it is less complicated than some of the answers above. Mike16 is right, we should simply be answering your question. The wire in question is the ground wire for the Brake Warning light on your dash. 12V is already supplied to the light and when this wire is grounded, the lamp will light up.

Most people, including myself, incorrectly use the term Distribution Block. It is actually a Pressure Differential Switch. It doesn't distribute anything.

The rear piston in a dual master Cylinder is called the Primary Piston. The front piston is called the Secondary Piston. The pressure (psi) from the Primary should always be the same as the pressure from the Secondary. This is true if the brake system is for drum/drum, disc/drum or disc/disc.

The Pressure Differential Switch needs to be plumbed into the Secondary brake line and into the Primary brake line, so that it can compare the two pressures.

The so called, distribution block is just the plumbing for the differential switch.
There is nothing inside the block except that double headed piece of brass and two springs. I'll call it a piston, but it is really half of the switch. The rubber "O" rings prevent the brake fluid from flowing from one side to the other. The orange wire I threaded in one hole and out the other, represents the Primary fluid path. The Yellow wire, the Secondary path.

If the pressure in one brake line is higher, the piston is pushed to the other side and the piston makes contact with the other half of the switch, which is the insulated pin that your wire is connected to.
The two springs are necessary to complete the ground circuit. because the rubber "O" rings act as electrical insulators.

I'm pretty sure this is the switch block from my '67. There is a later type switch in the image. It is a self contained switch. When the little nipple on the end is pushed in, the switch is closed and the ground circuit is completed through the body of the switch. This switch requires a "piston" with matching tapered shoulders in the middle, instead of the square shoulders of the piston in the image.
That switch is used in later switch blocks and in Combination Valves. I think there is another type switch, but I can't recall what it looks like.

How's that for un-complicating a very simple device with one moving part? ha