I restored my A/C system last winter when I installed my new motor. All lines were blasted, cleaned, and painted, condenser is new, drier is new (twice), compressor spins well with no excessive noise. All O-rings were replaced. But the charge only lasts about a week.

I pressurized the system with air (about 120#) and checked all the connections with soapy water. This showed no leaks. I gained access to a sniffer so I charged the system, ran it through it's paces (man this thing blows ice cold when it's working), and ran the sniffer around all the connections. No leaks detected.

I've been suspecting that it's the evaporator, since it's the only piece that's not been gone through. With the system off, I turned on the blower fan and placed the sniffer in front of the vent. It went nuts. Does this confirm my suspicion of the evaporator having a leak?

I think you answered your own question. Try it again with the engine off but the fan running, just after running the A/C. Some products of combustion can set off the sniffers. Having the engine off would eliminate that. There should be enough residual pressure for the refrigerant to leak without the compressor running.
Craig

If it's the evaporator there's a good chance you'll pick up a whiff of freon (or dye) at the evaporator drain tube as well as the vents.