"irollman".....As guessed, that is an air connection for one of the proportional braking systems like M&G or SMI Air Force One. If its been on there for many years, its obsolete. The first generation units used a "T" to connect to one of your brake cans (big metal can) on the back of both rear brakes. The DOT later frowned on that connection with just a "T" and the aftermarket braking system companies went to a safer solenoid system that wouldn't leak air if the brake line failed to the toad.
Sooooo.....as you get time and are doing projects, I would remove the line before it becomes an issue later on. It's a simple job. Originally, they just unbolted the metal fitting from the brake can, added a "T" and then bolted the original air line to the "T" and added the line you're seeing to the other side of the "T". Simply remove the two air lines, remove the "T" and screw the original air line back into the port where you removed the "T".