Why do I get a negative pressure at a high point in my system where an air valve and part-full flow would normally prevent that from occurring? Shouldn't the pump add enough head to push the water over the hill?
By default, pumps only consider the boundary conditions (reservoirs and tank elevations) in your system. So, the pump will add enough head to lift the water to the downstream known hydraulic grade (tank or reservoir) or in the case of a closed system, enough flow to supply the demands. It does not consider junction elevations in between. If these high points would normally have an air valve with part-full flow downstream to prevent the negative pressure, you can use the air valve element so that the upstream pump "sees" the high point and knows to add enough head to overcome it.
For any air valve that is expected to be open in this way, ensure that you select "false" for the "Treat air valve as junction?" attribute. See the below articles for further guidance.
Troubleshooting negative pressures at pumps, junctions, & other node elements
Pumping Over High Points
Modeling Air Valves At High Points in WaterCAD or WaterGEMS
Modeling Force Mains with Air Valves in SewerCAD (GVF Convex Solver)