NFC is a useful solution for this task. The NFC device makes it easy and effective to purge and braze. You can set the flow rate for purging and then switch to the brazing setting for a pre-set flow rate. This device reduces gas usage and prevents any air from entering during the repair process. To use the device, simply connect it to the outlet of a standard nitrogen regulator and use the 1/4 male SAE  connection on the outlet for standard HVAC hoses.