No need to waste precious R12. Just put a $8 can of R134A with UV dye into the system yourself. Turn the A/C on for about 1 minute to circulate, then shut it off and go searching with your black light. The R134A eventually evaporates out of the system, and there's no harm done.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280