Ive had to solder in fittings on copper heating lines (Propylene Glycol) what ive found is if your removing a fitting and you dont have enuf room to cut it back for fresh clean copper (stub coming out a floor joist or wall, the best thing is remove the old fitting by applying heat and working it off the tube, use a cotton chore gloves and quickly grasp the hot pipe and wipe off excess solder, let er cool off (I use a dish pan or bucket of cool water and dish rag to cool it down so I can work around it without getting burnt) and use the screen type sand cloth and work the remaining solder off till bare copper shows (remove as much trace of old solder as you can)
when repairing leaking lines, close supply & return valves to isolate the leak and drain off best you can, I use a wet & Dry shop vac to suck out any residual fluid in the line.
when soldering the center of a run, I'll solder one end the fitting first, then I apply a vaccume to the line (hook up a washing machine fill hose to a bib and suck on it till I can suck more and close the valve, I continue to solder the second leg of the fitting (as heat is generated it pressurizes the air within blowing out air thust a imcomplete solder joint)
Cheating- I cheat as much as I can every time using sweat in copper unions, theve made my soldering life soooooo much easyer!
Also I use the hose type torch body, having a pizio electric trigger is
I use MAP gas instead of Propane
carry plenty of paper towels to dry wipe the freshly sanded copper tube to remove that darkish grit from the joint