proportioning valve??

adriver said
The more I read Inline’s explaination, the more confused I get.
What they call a distribution block (250 on the right), I’ve always called a proportioning valve.
Regardless.
I’ve taken several of the ones on the right apart (250). I can’t really can’t tell how that switch works.
It’s got a flush Philips head on the body. That’s all I can see.
Otherwise, there doesn’t seem to be much about them to go bad.
It is all brass except for a spring and two little washers.
They usually aren’t bad.
There is an O-ring and a cupped seal on a plunger.
If you take the cupped seal out of the body (bore) WATCH OUT.
It will expand and you will play heck getting it back in.
If anyone does have a rebuild parts source, it would be nice to know.

Concerning the valves that were used on A-bodies, calling it a proportioning valve is technically incorrect. It is not a proportioning valve, no matter what anybody calls it. Think about it, what does the word proportion mean? It basically means "difference or differential". A proportioning valve regulates/limits/cuts down pressure, however you want to say it to the rear wheels. It does so because vehicles with light rearends or oversized rear brakes will tend to lock up and the back end will come around if you mash the pedal too hard.

What those valves are really is nothing more than a switch valve and distribution block. In fact if you look at the description on Inline's page showing #250 they don't call it a proportioning valve, they call it a distribution block. Some (like #250) also have 2 other circuits. The valve in the top that the line from the rear of the master cyl. screws into is a hold off valve for the front brakes cause disc brakes grap quick. The part in the lower section I'll do my best to explain later.

The switch valve circuit is there only in case something fails in either the back or front brakes. When something fails and that part of the system looses pressure the other side that has pressure forces the plunger over to cut off the supply of brake fluid to the bad side so you don't loose all your brake fluid. In doing so it allows the good side to still function. In addition to that the plunger contacts the switch to turn on the "brake" like on the dash to warn you something is wrong.

I'm not absolutely positive but I believe the lower section with the O-ring and cupped plunger is a residual pressure valve. It is only needed on a drum brake system (such as the rear wheels). Disc brakes apply more evenly and quicker than drums so an imbalance occurs when the 2 are used unless a residual pressure valve is in the circuit to cause the rear drum brakes to come on quicker. And you are correct about the cupped seal being a bugger to get back in if you take it apart. I wish I knew someone that sold a rebuild kit for it too because I have a valve here that's a good valve but I took it apart to clean it and the cupped seal was so swollen (probably from being in brake fluid all those yrs) I can't get it back in so the valve is nearly useless.

Hopefully that helps :D