A leaking copper fitting on a water pipe may appear intimidating but it is really nothing to be reluctant to fix to provide the right equipment is available. All that is needed is a gas burner torch and some solder and flux.
There are two reasons why a copper fitting will lose its watertight fit and begin to leak. It was either improperly soldered to begin with or has become old and worn out and possibly has cracked. In either case, the joint must be opened up and a new fitting put in place.
Begin by shutting off the water supply to the pipe and making sure the pipe has drained. Then begin heating the fitting with the torch. This will not take long as the melting point of solder is quite low. When the solder is soft separate the pipes from the copper pipe fitting. Be careful to use something to hold the part as metal will be hot enough to cause burns. Save the old copper fitting for scrap or discard. Once the pipes are separated from the fitting, wipe away any solder residue from the pipes while they are still hot. When the pipes have cooled take a piece of emery and sand the area thoroughly that is to be attached to the new fitting. This will prepare the surface and create a better bond between the parts.
Brush some flux on the ends of each pipe and place the new fitting over each end and cut a foot long length of solder. Begin to heat the end of each pipe, one at a time. Once the fitting has heated sufficiently run the end of the solder around the entire fitting. If the fitting is sufficiently heated the solder will immediately melt on contact. Repeat the process on the other side of the fitting.
Once the fitting has cooled, turn the water supply back on. The water entering the pipe will finish cooling the soldered area and it will now be safe to the touch. Now check it for any leaks.
If the area around the fitting still leaks, it will be necessary to repeat the entire process. If it is still leaking after a second application there may be an issue with the pipe and a licensed plumbing contractor should be called. However, most of the time the method applied will be successful.