My best guess is that poor original work opened up a leak in response to heat or mechanical stress working on the other line. You need to drain the system, pull the leaking joint apart, clean it up good, and resolder it. I have actually had a pipe pull clear out of a fitting when I tried to make something fit. Where you are working close to another fitting, you should wrap the other one in a wet rag to protect it from the heat.

You may need to give us some more details. It is a hot water line that is leaking now? How close is everything?

Hey Jet, You failed to tell us where the pipes were leaking from. A joint at a fitting or pinholes in the pipes themselves? If it's from a joint I would drain the system down again and resolder the joints. If it's pinholes in the pipe then you have electrolysis and you will have to ground the system and replace the faulty pipes. More questions? Just click on back. Cheers, Tom