Brake proportioning valve.

[ChevyRacefan]

Hello,
I guess this is a basic question , maybe not.
The proportioning valve/block.
Has the brake warning switch, and the ports in from master and ports out to the wheels and a thing screwed in the nose of the unit with what looks like a pin in the center of it ,.
What is it ,and what is it for, Thanks.

[katwalk]

If you have a leak in the front or rear and run out of fluid, there is a valve in that block that shuts off the side that is empty. When you push that pin, it re-centers the vale in the block.

[68 C10 Driver]

That is the metering valve for the front brakes.

[ChevyRacefan]

Originally Posted by katwalk

If you have a leak in the front or rear and run out of fluid, there is a valve in that block that shuts off the side that is empty. When you push that pin, it re-centers the vale in the block.

Maybe that is why after fixing the rear line, and trying to let the fluid gravity bleed, nothing came out.
Do I just push the pin in, and it self centers and fluid will start to flow to the rear

[custom10nut]

This is great information. Keeping this for later

[lousyflyer]

Its for bleeding the proportioning valve body and works quite well if you were born with three hands. There is a little tool you can get or make yourself to hold the pin in for you.http://67-72chevytrucks.com/vboard/s...d.php?t=722308

Also, every time you push in that little pin an angel gets it's wings.

[lousyflyer]

I almost forgot. If your brake warning light is on AFTER bleeding your brakes, depress the brake pedal FIRMLY and the fluid pressure will equalize and move the warning shuttle valve back to center, relieving the light switch plunger in the valve body and the light will turn off.

Knowledge about these trucks is best gleaned from old, drunk mechanics in bars.

[RichardJ]

The "Metering Valve", also called a "Hold-off Valve", may need the pin held IN or it may need the pin held OUT, while bleeding the front brakes. There are two types.

In operation, the Metering Valve remains closed under 50-60 PSI. When bleeding, with air in the lines and the bleeder open, there will be no pressure to open the metering valve and that is why the metering valve needs to be held open in order to bleed the front brakes.

The Metering Valve and the pin in the metering valve has no connection to the "Pressure Differential Switch" located in the middle of the "Combination Valve"

The "Metering Valve", the "Pressure Differential Switch", and the "Proportioning Valve" are completely separate items with completely separate and independent functions.

The metering valve pin IS NOT connected to the switch.



https://67-72chevytrucks.com/vboard/...in#post8093831

[70STOVEBOLT]

RichardJ how do you determine whether it needs to be pushed in or pulled out?

[RichardJ]

That is a problem. When they get old they get gummed up, stuck and hard to move.
That little metal clip used on the type that is pulled OUT, isn't able to exert much pressure, so you can see how little pressure should be needed to move the pin.

I've never had to mess with the old cast iron units. All the brass combo valves I have worked on had a pin that needed to be pulled OUT. When everything is working correctly, you can see the pin move OUT slightly when you have someone apply the brakes.

https://www.classicperform.com/Store...ucks/PV-MV.htm

If the pin is in too far to get a hold of and you think it is the type that should come OUT, the internals may be screwed up. CPP says their replacement unit should fit some factory combo valves. I have no idea if that is true.

[70STOVEBOLT]

I have the cast iron stock type pictured in post #8 above so I believe mine should be held in to bleed.