Hi, not only the type of copper pipe should be known, also the copper alloy of the T-joint. In general there should be no problem with brazing with a silver based alloy, provided the joint itself presents capillary clearance. The difference is the amount of silver, the alloying elements and the solidus and liquidus temperatures. Use of fluxes is almost always recommended: type can be as suggested by filler manufacturer. To determine previous joining material one should obtain a small chip from a scrap joint and send it for analysis. Does it melt when touched with a hot soldering tool? The cheepest qualitative check available is probably X-Ray Fluorescence. However it seems you should not be overly concerned with old joints, unless your new joint is very near to an old one, so that you might fear to heat up the old too much when brazing the new. You can keep old joints cool by submerging them in water. Good luck!