Just for completeness for this thread, for others who read it, 2 other points I've learned on this site which really helped me get leak-free joints:
1. Make sure that the system you are soldering on is open to the atmosphere, or else the heated air will push solder out of the last joint you solder and you will keep wondering why that joint keeps going bad, despite cleaning/fluxing/soldering correctly.
2. Heat the fitting, not the pipe.
1. Make sure that the system you are soldering on is open to the atmosphere, or else the heated air will push solder out of the last joint you solder and you will keep wondering why that joint keeps going bad, despite cleaning/fluxing/soldering correctly.
2. Heat the fitting, not the pipe.