Hi, 1995 2500 with a 5.9/nv4500/np241. this line is under the driver's door, outboard side of the frame.

 

That's a brake line...... Probably was supposed to go to the rear brakes..... Follow it, see where it comes from, and where it used to go.....

 

I haven't had time to follow it (no second person to wiggle it), but from where I can follow it (it goes behind the fuel tank), it sounds like it's the rear brake line. why would someone replace the line and split it like that?

 

Probably rusted out and they used a compression fitting to replace it. I have no idea why the left side has threads on it

 

Ok, so that line has a plug in it, too, makes me wonder do you have rear brakes at all? Because that line looks to be going to the master cylinder...

MoparFanatic, I had a two piece rear line on my CCLB, though I can't remember where the extension was located.

 

Would make sense for a CCLB. Duh! I had a moment there lol
I just assumed it was a short bed. You know what they say about assuming. Most likely OP does not have rear brakes.
If it is suppose to have the threads, did that mean someone started the repair and didn't finish?

 

I'm just wondering if someone had a hydraulic brake controller hooked to the master cylinder. I haven't seen one of those used in years as solid state ones have gotten so reliable. I've removed the lines for them and plugged the fitting with a metal cap.

 

I'm not familiar with them so I can't comment

 

Up to the 1970's, that was how you engaged the electric brakes on a trailer. A line was run off the master cylinder. A metal line was then plumbed down the firewall and up through the floor to a controller under the dash. When you pressed the brake pedal, it moved the lever on the controller. On inertia switches. You could engage the trailer brakes with a lever like the modern electronic units. Th last time I saw one was on a 1988 W150 that has pulled a horse trailer.

 

Now that is interesting, and would make sense why there is a compression fitting and a threaded end