I'll move this thread to the MKII section where it belongs.
No, a leaking slave isn't going to ruin your transmission but excessive wear on the clutch and damage to the clutch plate and flywheel are possible. The slave cylinder is a very simple device, I'd rebuild it but they're cheap so just swapping in a new one shouldn't break the bank either. If the slave really is where the leak is then yes, replacing it should prevent and *new* damage.... It won't undo any damage which may have already been done though.
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