How to create a vacuum pump in open Modelica

Question

I need to simulate a vacuum system. I am facing multiple problems : first I can't find a way to make the pump flow dependant of the pressure inside the chamber. Secondly, with a constant flow, the value I get is highly different from what I have inside the actual vacuum system. Did someone already have this situation and can help me out?

I tried to add pipes, I used a sweep volume to mimic the pump and I tried using a combitabl to use the characteristic curve of my pump.