I have noticed that with a few of the compressors I have had to changed on on a few Armstrong package units, and a condenser coil I had to change on a Carrier RTU, I have ran into problem of having partially plugged evaporator coils. Now whenever I braze, I always have a nitorgen purge going through the system to reduce the chance of oxidation inside the copper lines. I was talking to the tech who either changed the compressor on the Armstrong systems or repaired the evaporator coil on the Carrier RTU and he indicated that he never uses nitrogen purges when brazing. I am thinking that maybe these previous compressor changeouts and repair resulted in oxidation forming in the connection after the liquid line dryer and before the metering device. I have been only doing this for about two years now, so any thoughts from the ones with years of experience would be helpful.