I am guessing, you are trying to build a valve block that will achieve any given pressure at its outlet?
I think the best way to model the "behavior" of this valve is to add some control elements to a variable orifice. The orifice will open wider if the outlet pressure is too high, and vice versa.
If the set pressure comes from a pilot pressure signal, you can also use valve actuator blocks to reference the outlet valve pressure, so at equilibrium, the outlet pressure will match the pilot command.
Sorry I am making lots of assumption with my answer. More clarification on the question would help.
