What is this under master cylinder

[makalaniman]

Aloha all...
Replacing my master cylinder and wondering what this solid metal block/coupler-looking thing is...
There is a wire harness that connects to it.
I have the vacuum assist brakes.
What is it?
Thanks!

 

[makalaniman]

2003 yukon 4.8L

 

[mikez71]

That looks like a brake proportioning valve for your front/rear bias.

 

[makalaniman]

Thanks!

 

[OR VietVet]

Be aware, you have some rusted lines there. What do the lines under the vehicle look like?

 

[makalaniman]

Thanks for the concern. I replaced all the lines a couple years ago, and stopped at the proportioning valve. I'll replaced the the line between the valve and the cylinder when I swap the cylinder this project.
Do you think I should replave the proportioning valve too, or should that be fine to reuse?

 

[OR VietVet]

Typically, the proportioning valve is rock solid and hardly ever goes bad. If it was me, I would search for a Genuine GM or AC Delco part and replace it while I am in there but if you don't, you should be ok. Inspect thoroughly while apart.

Shows "Out of Stock" at RA but if you ask, they will notify you when gets back in stock. https://www.rockauto.com/en/moreinfo.php?pk=1568554&cc=1415540&pt=1888&jsn=962

You can post the last 8 digits of your VIN and maybe @915_Tahoe can supply a list of places that show NOS part on their shelves. He has helped me personally with a NOS list.