Kelsey-Hayes proportioning valve for A body

I think the name of that fitting is called a Residual Pressure Valve. It's what designates the whole shebang as being for front disc brakes. The whole brass part below it is divided into two halves. Top part is above the mount bracket, with 1 line into the part you circled and 2 lines out going to the front disc brakes. Bottom part has 1 line in and then 1 line out going to the rear drums. Unless I misunderstand them, the only difference between the disc and drum blocks are the valve on top. I have a diagram drawn on a piece of cardboard and stuck up in the shop showing where each line goes because my CRS makes me forget! :realcrazy:
Residual pressure valves are located in the master cylinder as required. Ie; 4 wheel drum master has a residual pressure valve each located in the port of the master right behind the brass line seat. A disc master has only one residual pressure valve located in rear segment port, you don’t want residual pressure on disc brakes.
the thing you circled is just and adapter to accommodate the brake line. You take the adapter out to get to the guts of the valve.