Welds made without the passage of nitrogen into the tubes lead to the formation of russets, which are not easily removed by R141b;
Nitrogen is much cheaper than R141b. There's no reason not to use it.
R141b should only be used for final cleaning.
Copper tubes should have the burrs removed and the ends sanded.
Whenever the system is opened, the dryer filter should be replaced.
After engine burn, change the oil from all circuit compressors, filter drier and install post-burn filter in the suction if necessary. The acidity resulting from the burn will burn other compressors that are interconnected in parallel if nothing is done in the system.
