应该是三个步骤

1. Purge the line with nitrigen while brazing.  This would guarantee inside of the copper tubes will be clean because there is no oxidation.

2. Pressure up with nitrogen to check any leaks.  Wait for an hour, if the pressure does not change at all, there is no leak. 

3. Vacuum the system to less than 500 microns.  This could take another hour.  

Nitrogen is cheap, but time is money. You can figure it out what is a reasonable price.