Find path upstream with weight in geometric network (ArcGIS)

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Hi everyone, I have an issue about using weight in geometric network in ArcGIS Desktop. In the example below, I have a simple network with a source & some junctions. And it has a loop through B-C-D-E-F. I'm trying to use the Find Path Upstream function to find the path from junction A to the source, i've created a weight based on the length of the edge feature, but it seem doesn't work. Can anyone tell me or show me:

• How can I set up a weight to Find Path Upstream from point A to the source through the shortest path (which means A-B-C-D-E-Source in this case)?
• Which weight should I use & how can I do it if I want to find path from A to F to Source?

Thank you very much for your helping