Dave, thanks for all of the info so far!
So 3/8" fittings usually means 3/16" line? Is that O.D. or I.D? Because there has to be a flare, does the wall thickness always have to be the same? Otherwise, for the same I.D. the O.D could always be different or vice-a-versa.
I would have assumed that, except for there are adapter fittings everywhere in this brake system, and I have no idea why. I think these brakes were all custom (from Featherlite) but it still doesn't account for all of these adapters. This system has not been touched (as far as I know) since purchased new. This line goes from near the master cylinder to the t where it splits off to go to the wheels. At the 'front' it's 3/8" fitting and at the rear it's 7/16" fitting. But at the front there is immediately an adapter which is also 7/16". If 7/16" was needed at both ends, why not just use it at both ends? It was clearly an option at the back end of the line.
So I did talk to one place yesterday, Ernies Automotive Services on East Main. They said they do custom hard brake lines, but as soon as I described the line they called it a hydraulic line and said they don't do those. One of my coworkers said they used to make lines like this, but larger, for farm equipment at the NAPA he worked at. Most of this line is that black, rubber like flexible line until the ends, they there is a short, few inch, metal portion crimped onto the end so they can have a flare in the metal pipe and a fitting that swivels so it can be tightened down.
Anyways, I will try them out and see if they make this type of line. It's no problem taking it off and bringing it in.
This stuff
http://www.goodyearep.com/uploadedImages/Product_Categories/Original_Equipment/Products/Hydraulic_Brake_Hose/Hy%20BrakeLG.jpg