It sounds like the weld (braze more likely) has a defect. You might see if there is a shop local to you that can re-braze it for you.
Also, really, really, really closely check both the inside and the outside copper washers that seal the Banjo Bolt and the Oil Line to the Cylinder Head. Sometimes leaks like this can be very deceiving in their origin. A pin hole can be squirting an extremely thin stream that when it hits something nearby it spatters. The spattering doesn't have to be dramatic, it's just what is most visible. The thin stream can be very difficult to see all by its lonesome. Furthermore it can easily be obscured by the spattering. The result is that it (falsely), looks like the leak, is where the spattering is occurring. If you can clean the area spotless with brake clean, carburetor cleaner, etc. and (this is actually quite important) while you are watching it intently with a bright light (even in daylight), have someone else start the bike.
If these are not options that are available to you; I've got a good Oil Line you can have for $10 + Shipping.
Last edited by Bluehighways; 06-03-2017 at 04:32 PM.