Is there a book, webpage or document somewhere that'll tell me how to safely recharge my R134a A/C system?

Bentley & Haynes are very silent on how to do this. I've seen general A/C books, and it looks like the various recharging products at the autoparts store have generic instructions on them. But if there's a Volvo-specific document out there, that'd be "cool" (haha). Or does it really matter?

The system was "professionally" converted about 3 years ago (new compressor & all) and it worked great but it has a slow leak somewhere. At the time, they couldn't find it. So I think I may just recharge it and not worry about it again for a couple years, since it's leaking the cheap stuff.

Are those "stop leak" products worth anything? Or do they just gum up the system unnecessarily and make it harder to fix in the long run like Fix-A-Flat type products?

THANKS!