If you say the car has factory R134a, it does not use a standard schraeder valve like those used on tires. I don't think a R134a valve stem can be replaced the same way; on my wife's van, when the valve started leaking, they had to replace the entire line since they said the valve was not repairable. Thankfully the repair was done under warranty.

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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