That don't look so bad of a spot. Some nitrogen and soapy water will confirm the leaky spot. You do need to use a purge gas while brazing anyhow. A inspection mirror works wonders too see completly around tubes in tight spots. If it is a rub through from the vibration of that poorly mounted sensor you can flow some non-silver bearing brazing rod to patch a pin hole. Silver bearing flow out to much. Clean surface and no flux needed just copper/phosphorus rods,they do this nicely. But their liquidus temperature is over 1700f to 1800f so a two gas torch works best. Propane/O2.........mapp/O2.......Acetylene/O2. If a rod melts but the repair looks gooby or didn't flow you only got to the rods solidus temperature.
Somehow I knew you had a pic.
