This is how we do test for purge system leaks we use SDS to activated charcoal canister shut off valve then we remove purge line to regeneration valve and attach vacuum tester with three way hose one way to vacuum gauge one way to purge lline one way to a air condition vacuum pump apply of 25 mbar and pinch vacuum hose "After one minute not more then <5 mbar of vacuum loss" this way you test fuel cap fuel tank charcoal canister,shut off valve and vacuum line in between but not the regeneration valve ! also you can compare intank sensor reading on SDS to your vacuum gauge should be the same. hope this help. if you have more then 5 mbar loss in one minute you have a problem. have to isolate the leaks.

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MB STARS Master Guild Technician,17 years MB tech
14 years independent shop owner