Another often-overlooked source of vacuum leaks in the evap system is the fuel cap seal. When the purge valve operates it places the whole tank under a slight vacuum, if the cap seal leaks, the vacuum won't happen and the sensor won't register a "purge".

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

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

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]