If the aim is too model circuit, you need to create a solver in graphs.

The graph would be composed of your pneumatic elements. If you model all your pneumatic element with linear law you can after easely  create a system matrix to know  the pressure and flow in all the circuit. If you dont know how to programme this you can do analog modeling with electric composent where for exemple a tip is a resistance with U=dP (pressure difference) and I=N (flow). This sort of hypothesis work as long as you model simple pneumatic composent in steady state and with the hypothesis of laminar flow.

If you are not in laminar flow you can try to design law for every composent by doing simulaion with Fluent for exemple or with experimental test and after do interpolation laws. If you buy the composent you want to model, you can find some solutions in the data sheet. But the problem after with non linear model, it will turn very hard to model circuit after because you will need to use an iterative methode as newton Raphson....