If you do have to extend the copper line, and you are brazing copper to copper, stay away from the silver solder. Grab a stick of Phos-Copper with some silver content.
I use Stay-Silv 15. It seems like it should be called silver solder, but its not. Silver solder will work, and do great. And if you are connecting anything other than copper to copper, then silver solder is what it will take.
But, silver solder needs flux to ensure a good joint, and I don't like flux in my gearboxes.
Phos-Copper is self-fluxing on copper. No flux needed. Phos-Copper 0 is good stuff and cheaper, but needs a tight joint for strength and can be stiff and brittle.
The 15% silver mixed with the Phos-Copper lets the joints be sloppy and will cause it to do good filling. And be ductile afterwards. Plus its very easy to solder with and quite forgiving.
As a side note, its what all my outdoor ground connections are made with. Living in the lightning capital means you must have good solid low resistance connections on all the dozen or so ground rods I have.