Low Pressure Sanitary Sewer - Negative Pressures

I'm modeling a low pressure sanitary sewer. The goal is to see how the system reacts as homes/grinder pumps come online. The issue I'm having is with only a single grinder pump operating to start in a steady state analysis, the system HGL goes negative part way through the system. See engineering profile attached. When I model the system with 3 grinder pumps online at the same time, there is enough flow so that the HGL is positive. It's as if the the single pump is trying to fill the system and cannot. Any insight into why SewerCAD reacts this way would be helpful. I have also attached the model. Currently the additional pumps controls are set so the on/off HGL settings are the same so that they don't come online.

Profile Report_Engineering Profile - Profile - 1 (Low_Pressure_Sanitary_Sewer_1PUMP.stsw).pdfLow Pressure Model.zip

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  • Tom, this lower pressure sanitary sewer is discharging to a gravity sewer manhole modeled as an outfall in a section of pressure sewer that is constructed to the north of the first section from about STA 15+00. The first profile is not discharging far to the right,  but flow heads to the north at J-25. I have attached the additional engineering profile for that section & a larger version of the first profile.

    The notification I get that is of concern is "Pressures below the lowest physically possible pressure in system at one or more time steps".

    There is an air valve in the first profile near where the pressure goes negative, which I'm currently modeling as a junction. 

    In a modeled low pressure sanitary sewer with pressure pipes that have high points/air valves, when you say "if there are downhill slopes with an air valve at the preceding high point, the downhill section will drain and will flow partly full." - how does the model show that? Is it represented by what I'm seeing?

    Profile Report_Engineering Profile - Profile - 2 (Low_Pressure_Sanitary_Sewer_1PUMP.stsw).pdf3718.Profile Report_Engineering Profile - Profile - 1 (Low_Pressure_Sanitary_Sewer_1PUMP.stsw).pdf