I recently repaired the AC on a 97 Spyder. The liquid line had a hole corroded in it
about 8" East of the battery. (8" to the right, in a bend on the firewall). I replaced
the liquid line, evacuated the system for ~60 minutes, and let it sit for ~30 minutes.
 It held 29" of vacuum. I charged the system and it worked great. A week later, I had
the car for another problem and the AC worked perfectly.
 Now, 4 weeks later, the AC is dead. The system charge is Zero. 0psi. No fluctuation
with temperature, just NADA.
 That sounds like a big leak. Easy to find, right? Nope. I pull a 28"vacuum and it
holds for an hour. (I'm not more patient than that).
 On the previous repair, I injected UV dye into the system. There is no trace of UV
dye anywhere. (Even in the evaporator drain, before you ask. )

I'm suspecting the compressor shaft seal. They usually don't reveal dye unless they
are leaking pretty bad.
 Any thoughts or opinions? Experience is very welcome!!