OK, I'll bite. Why are you trying to add R-13 to a R-12 system?

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible

The system could easily have R-134 in it without the conversion fittings or labels for R-134. Be careful.

when we bought the car, we were told that it was converted, but the past owner was kind of....ghetto lol so I'm not sure if they actually did it or just put the fittings on and said it was converted.

But since I thought it was converted I went ahead and tried refilling it with R-13 but low and behold it didn't fit. So I'm wondering what's going on here LOL

I'll post a picture of the fitting.

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76 MG Midget
78 Datsun 280Z
86 Peugeot 505
01 Subaru Outback

“Owning a car is like owning a dog. Every now and then, it craps on the carpet — but it’s still your dog.” - Jeremy Clarkson

OK, so I assume you're talking about R134a, not R13.

Are you sure that you are trying to connect the R134a to the suction side and not the high pressure side?

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible