Successful copper pipe sweating

Ya gotta do everything completely. No shortcuts.

Make sure all joint surfaces are completely clean, inside & out. Sandpaper and wire brush until they are all bright and shiny.

Apply soldering flux to cover each clean joint surface. Here is a spot where things may fail. Flux works by chemical reaction. If your flux is old and no longer active, it may not work. I've had several joints in an assembly fail to draw solder. After recleaning and then changing to fresh flux, they drew solder just fine.

If there has been water in the lines, you need to keep it away from the joint. Water from higher points in the system will continue to seep downward and keep the joint from heating properly. Stuffing some soft white bread into the pipe toward where the water is coming from will solve the problem long enough to get the joint to seal. The bread will dissolve when water runs back through the line. It may clog the aerator in a faucet, so you may have to clean that out after.

Assemble the joints and then apply heat. (MAPP gas works much faster than propane.) The solder will flow towards the heat source, so after heating the entire joint, move the heat back from the joint so that the solder will be sucked into it. Make sure this has happened all around the joint.

Keeping the flames off combustable materials, like studs and insulation, is important. I've used ceramic tiles propped behind the joint to accomplish this. There are flame shields made for this purpose that are better. Ceramic tiles often crack and fall apart while you are soldering.

Let the joint cool off some before letting water back into the system. I've heard that cooling the joint too quickly can crack the solder.

If you are patient and follow all these steps you should have joints that don't leak.