My 91 300E is on the original evap as well. Although the system does leak, it only requires 1 or *maybe* 2 cans top-off per year (here in TX you pretty much need AC from April to October, hence the *maybe* 2 cans). That equates to only $15 - $30 per year so I just top off from my supply. If it ever gets worse I'll start chasing the leak down...

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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