Whatever the cause, a leaking copper pipe has to be fixed. A number of temporary repairs ranging from epoxy putty to rubber-enclosed clamping systems are available, and each has its strong and weak points. They fix the problem temporarily, but eventually a permanent repair must be made. Replacing the section of copper pipe that is leaking is the best permanent repair available and requires soldering a new piece of pipe to replace the leaking one.
Turn off the water at the main shutoff valve. Open the faucets in the house to relieve pressure and drain the pipes of water. If the pipe being worked on contains water, you will be unable to solder the copper pipe.
Cut out the portion of the pipe that is leaking with a pipe cutter. Have a bucket ready in case the pipe contains more than a few drops of water. Turn the handle to open the jaws just enough to slip the cutter over the pipe so the two rollers are on one side and the round blade is on the other side. Tighten the pipe cutter and rotate the cutter around the pipe once. Tighten the cutter again and repeat this process until the pipe is cut. Cut the pipe in two places at least 4 inches apart on either side of the leak.
Measure the piece of pipe just removed and subtract 1/2 inch from the measurement. Measure and mark a new piece of pipe and cut it off 1/2 inch shorter than the piece removed.
Polish the ends of the new piece of pipe with emery cloth. Wrap the emery cloth around the pipe end and twist it back and forth until the pipe surface is bright and shiny. Polish the ends of the pipe from which the leaking piece was removed the same way. Polish the insides of two fittings with the fitting brush. Insert the brush and rotate the fitting until the inside is bright and shiny. Do both ends of both fittings.
Coat 1 inch of all four ends of the pipes with solder paste using a fitting brush. Coat the inside of both fittings at both ends with solder paste. Place the fittings on the pipe where the repair is being made. Add the new piece of pipe, fitting it into the two fittings.
Place a heat shield between the fittings and any nearby flammable material such as wood framing, flooring or wiring. Unroll about 12 inches of solder from the roll but do not cut it.
Light a propane torch with a torch striker, match or by squeezing the trigger. Place the bright blue internal part of the flame on the fitting. As the fitting heats, touch the solder wire to the juncture of the copper fitting and pipe. Heat the solder with the fitting, not the torch, keeping the flame away from the solder. When the solder begins to melt, rub the end of the solder wire around the circumference of the fitting at the juncture of the pipe and fitting. Do this at both ends of both fittings.
Shut the torch off and let the joint cool several minutes. Wipe the fittings and the pipe with a damp rag to remove oxidation and dripped solder paste. Turn on the water at the main shut-off valve and let it run a minute to flush out the pipes before you close the faucets.
Buy couplings with a stop in the middle and avoid styles that have no stop. This ensures both ends of the pipe are properly fitted inside the coupling.
Don't heat the solder with the torch. The resulting joint will fail quickly under pressure and result in a free-flowing open pipe. Always heat the fitting, move the torch away from the juncture of the fitting and pipe and feed the solder into the juncture.