What is it?

[Mertz]

This is installed from the master cylinder to the junction for my brake lines. It is a 3/8” pipe vs the 1/4” brake lines. Is it some sort of old proportioning pipe in lieu of a valve. 

[P15-D24]

What exactly are you asking about in the picture? 

[Tooljunkie]

No proportioning at all. Requires a dual port master cylinder

[Merle Coggins]

The supply line from the master cylinder to the junction block is larger than the lines that run from the junction block to the wheels. Think of it as the main artery that branches off to the smaller veins that feed the various organs. It should have a loop in it to provide a little bit of flex since the master cylinder is attached to the bell housing and will move with the engine on it’s rubber mounts, whereas the junction block is mounted rigid to the frame. 

[kencombs]

Is that NiCop or just plain copper?  if the latter please consider swapping it.

[Mertz]

It’s pretty stiff so I’m sure it’s not pure copper. There is some nickel showing thru when I cleaned it with a wire brush.