brake junction block?

[rustyMUD]

The truck in question is an 81 GMC 3/4 ton with a 292 straight six.
I was doing some brake work recently and came across a strange fitting on the brake lines. The fitting is made of brass, and threads directly into the master cylinder. The normal steel brake line then threads into this fitting. On the top side of the brass fitting, there is a steel hex screw which plugs a hole that is smaller than the others which go into master cylinder and steel line. I figured this might be a junction block of sime kind, but having male and female ends, plus a smaller hole....well it has me stumped. Also, what whould something like this be doing here? Why not just hook up the brake line straight to the master cylinder? The steel brake line appears to be the same size and thread type as the brass fitting that goes into the master cylinder.
I know that pictures would help but I'm without a camera. In summary, any ideas as to what this brass fitting is, and what it's doing attached to the master cylinder?