There is something with that, I think it can size based on the connected loads, but not show flow in the System Browser. I'm not exactly sure, but I know there is something with the Sanitary that doesn't work, so you are not the only one. Maybe it is expected that there is NO equipment on the downstream end, although you may have a sewage pump sometimes.
On the same note, I had tried to "Finish" a Domestic Water System with a "Meter" to get the building Demand as calculated in the System Browser, and I could not get it to work.
I too found myself trying to close out a Cold Water System, and what I found was a family in the library: Mechanical Equipment / Pipe Connector - Domestic Cold Water - Plumbbing Equipment. Load it, edit Family, set the connector to be "Calculated" and "Loss Method" as "Specified". Hook it into a well connected cold system and it closed up the system giving me the overall flow value. There is a similar family for the sanitary that i will check next.
I also found some PipeAssessories:Water Meter based on HERSEY METER at Revit City. I had to tweek the connectors slightly, mainly matched up how the PipeConnector family previously mentioned was setup. It closes up the Cold Water System to where there are no more 'warning triangles' when I use the "Check Pipe System'. And yes, the System Browser appears correct to me.