It's fairly easy to do, on a larger system you can just leave one end of the tubing open while purging. On a residential condense change out, remove the schader cores, connect a hose to the regulater an set it so very little nitrogen is coming out (you don't want to pressurize the lines too much or it will be hard to braze) connect it to one of your lines and wait a few minuites for the system to purge, make sure you have flow exiting the open scrader, and begin to braze. Leave flow while the lines cool. When done use nitrogen and trace gas to leak check. Replace scrader valve cores. Leak check ok, purge nitogen out, vacuum to desired level and open valves on unit. At least you have not introduced any contaminants to system. Added time on this job 10, mins.