Set new air conditioner and run new line set. Silver solder flowing nitrogen with Victor Edge series regulator. While soldering with rags to protect the service valves it takes a lot of heat especially on a 7/8" line. Make sure the rags are rung out well and wrapped tight on the valve as far back from the joint you are working on or you will have the problem with the steam blowing out the torch. The heat needs to be just hot enough for the size of the line you are working on. With the flame too close to the joint with too much heat you will burn a hole in the pipe. The blue cone is the hottest part of the flame and will heat it up quick if that cone is right on the metal. Every situation calls for different torch movement if the flame or metal is too hot or too cold. Once the whole joint is heated up you will be able move around the joint and the solder will flow and you can pull it into the joint with the torch.. Or you can just skip the rags and end up with a leaky service valve and a unit that needs charging every year like so many. Pressure test with modified Victor Edge. We pulled the safety relief valve and added a tee with a high pressure port access for pressure testing. Before that it took forever to build up pressure for testing. Vacuum and charge system.