How do I remove flux residue from inside copper pipe when rinsing with hot soapy water has not worked?

if flux used was not water-soluble, it's most likely water washable. this means that you need to run a detergent through the lines. soapy water can work but stronger detergents should be used like laundry or dishwasher detergent and make sure you run very hot water through the lines. a good way to make sure the detergent you chose is working is to test it out.

how to run hot water through the cold line? that's a different question.