You have a bigger leak than just schraeder cores. Likely the shaft seal on the compressor or one of the MANY O-ring connections in the lines. Time for a full R&R on that system, there's not enough gas left in there to make it worth your time to save, if it won't trip the compressor on, it's basically flat.

For what it's worth, the R12 fittings are the same as R22 and R410a. Get a standard 1/4" valve core tool from Fleabay or Amazon for under $10 to your door, it'll fit the R12 ports perfect.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)