Have been reading a spate of posts mentioning the brass "proportioning valve". I happen to have the very brass distribution block originally installed on my 69. I have had this car since 1972 and have no reason to suppose it isn't original. A couple of numbers are stamped on the side - 8 296. That's all.

I pulled the bracket off the back, just in case there was some access covered up there, but there is not.

From the front, the electrical switch pulls out pretty easily. It slides in an insulated shaft and presents some 10 megohms of resistance to the insert, so pretty much an open circuit to the light bulb on the dash.

Inside, installed somehow is a shaft that I assume slides back and forth to make electrical contact with the little piston on the electrical post. You can just see it in the picture.

If I blow compressed air into either supply side, the air only comes out the ports connected to that side of the MC. So brake pressure is probably pretty well isolated front and back at least at the low pressure I can supply. I presume the way this works is as has been described by others, a little slider that makes contact with the plunger and supplies a ground if it moves off center. I can't make it move with a pair of tweezers so the tolerances are pretty tight in there.

Now, hydraulics 101. Pressure is measured in pounds-per-square-inch and area is of course square-inches. In a hydraulic system pressure travels at the speed of sound in the fluid, and is the same everywhere once equilibrium is reached. Orifice sizes do not matter at equilibrium. So the force exerted at the caliper pistons is simply psi X area, i.e. pounds of force. Simple enough units. The area of the rear pistons is a fraction of the front pistons and that is how brake bias is built into the system. Absolutely straightforward engineering.

Furthermore I have measured the hydraulic pressure in my system and it is the same front-to-back. The proportioning is 1:1. Even the time to reach full pressure from videos I took is the same front and back, indicating there are no other dynamics at work.

Here are the parts in the distribution block. I challenge someone to show me where the "proportioning" takes place. There is none!

 

Evidently, the purpose of the device is to operate the dash warning light in the event that either the front or rear circuits leak and cannot achieve the same pressure as the healthy circuit.
This may not be evident by pedal feel as the healthy side will retain its normal pressure against the pedal and your foot.

There will be some design incorporated into the switch piston and how it interacts or valves between the two sides that allows it reset to centre once the side that went low pressure has been repaired. Because the reset instructions are to "stamp the pedal" or "push the pedal swiftly" or some similar description.

Proportioning, as you explained is built in to the system by piston size between front and rear calipers.

 

It's my understanding that the brass block had a proportioning piston starting in about 1979. I welcome any needed correction to my recollection.

 

Yours was manufactured the 296th day of 1968. Late October. I would think also that it is the original on your 69.

 

Appreciate the authentication of the part. There's a #223 in crayon on the rear of the passenger compartment wall. Sounds about right, mine was built in late 68 sometime. Thanks 62

 

Interestingly, that 223 is no date at all, just a sequence number. As St. Louis built these, before they assigned a vin, they just sequenced the assigned job numbers 1-500, and then started over. So the 223 means it came after the parts that were assembly 222, and before 224.

 

You have the 'normal' distribution block for that model year. It is there to isolate the front brakes from the rear brakes [or vice versa] in case of a significant brake fluid leak. Thus, at least one-half of the system is still operational; and when the shuttle valve in that block shifts from its centered position, the BRAKE light on the speedometer is lit (to tell you there is a problem with the braking system).