If the tech who uses "stop leak" is the same tech who told you to vacuum the a/c system as part of some sort of maint. regimen, I would find another tech.
Use the search facility here. You'll find more than one negative post regarding a/c "stop leak" products from people here who do this for a living.
a/c "stop leak" products are nothing more than snake oil. Not only will they damage a/c service equipment, but eventually, this stuff will deep-6 your a/c system.