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

[sean1969c10]

So just a general related question, on these trucks the brake light on the dash will not come on when the ignition is turned on, correct?

[RichardJ]

As I said, I use the term distribution block even though it isn't. I do not use the term Proportion Valve when talking about a Combination Valve.

A Combination Valve has three devices COMBINDED into one. Combining the three saves a lot of plumbing space and expense.
The first image has a newer Switch Block, a Proportioning Valve and on the bottom a Metering Valve.
The late '60s Camaro, Chevelle and maybe others with disc brakes, used all three of these devices and associated plumbing. That fixed Proportioning Valve was also used by Mopar, AMC and maybe others.

The second image shows the contents of a Combination Valve. You can see that the Switch piston has the tapered shoulders to push the nipple on the switch, inside the switch and close the ground circuit.

The third image is a Proportioning Valve. Fixed
The fourth image is a Proportioning Valve. Adjustable
The fifth image is a Combination Valve. The older type used on these trucks.

[AnotherWs6]

Thank. Good info. Not sure if it solves anything for me, but I was just looking for info anyway.

[RichardJ]

>>Not sure if it solves anything for me<<

Other than saying "I have wonky brakes" , you haven't said what you changed from the original photos and what problems you are having.

[mike16]

well it certainly answers the question fully and completely regarding the wire. Well done and your pictures are better than your excellent explaination.

I always like to answer the question fully and completely. because for every one person asking the question there are several mor who can also learn from the answer. complete answers are best. I like explainations that lead to more questions.

the metering side of the prop valve has I believe, a reset plunger type button under the boot. I have seen alot of these as well but not all prop valves have this. and prop valves can be fount on drum/drum, disc drum, and disc disc set ups. especially on wagons and trucks where the brake pressure on the rear brakes need to vary based on the load. in the diagrm showed above.

[old Chevy guy]

So am I the only one who thinks that is the wrong master for front disk /rear drum brakes? Maybe I'm wrong but I think this is where your wonkyness could be coming from.

[FirstOwner69]

I think we are all getting a bit confused. In his first post the OP asks if his setup is correct for 4 wheel power drums. In my opinion, the answer is yes.

In post 10 he says he bought a full disc brake kit. We don't know if he has installed it, nor do we know if "full" means all four positions or front only. We are also still waiting for a definition of "wonky". If it is a front disc/rear drum kit, I agree that he probably needs a different master and definitely a proportioning valve rather than distribution block.

We also don't know if the truck is a 4WD or 2WD, 1/2 ton or 3/4 ton. These things could also make a difference in terms of hardware and plumbing, especially for discs.

[AnotherWs6]

It is a 1/2 2wd Suburban. I bought a full 4 wheel disc brake kit which I said "came this week" so of of course I had not installed anything and still haven't. And a someone pointed out, I was just asking what the wire was for. I did throw "wonky" in there eventually, here is what I mean very quickly. Brakes lock up at low speed with minimal pressure. Impossible to modulate, they just GRAB. Slowing down from highway speeds the pedal is very soft and requires a lot of travel to even begin getting any bite. Once pushed far enough pedal firms up and it is possible to modulate the brakes, but also possible to lock them up easily if a quick slow down (someone cuts in front of you is required. Adjusted all 4 brakes and drove around, was able to get them to perform and modulate decently though not great. Drove the truck again and it once again wanted to throw me through the windshield. Parked it. Drove it again and was able to modulate the brakes again. None of it makes sense.

[AnotherWs6]

I inspected all of the brakes. Everything looked great.