That is an old style air pack. The problem with making one with copper is the bending. It is extremely hard to bend tubing of that size so you will end up using 2 elbows and have to fit the lengths carefully. If it were me, I would get another J Tube. I had one from an airpack that could not be rebuilt but I used it on the next one that came onto my bench so I cannot help.
One of the reasons I can see for opting for an oem part is that it is very unusual for this tbe to break and it is likely due to damage from an external cause that will necessitate the replacement of the part. The military uses a rigid, thick walled tube as brakes are nothing to take a care free attitude about and since this is how your truck stops , I would not risk my life, or yours by cutting a corner. If this tube fails at road speed you have a 14000# steel projectile that is not going to stop any time soon.
This is just my opinion and you can do as you like. I would add that for the sake of the hobby and the potential negative fallout that could occur if a part you fabricate (instead of an oem part) fails you are simply helping the opponents of private ownership and operation of MVs in their quest to ban these trucks. Aside from that any part you make is your personal liability to deal with if anything bad happens even if it is not directly related to your part.