help servo pnematic system model (using governing equations )

hi i am using simulink for simulating a servo pneumatic system model using the governing equations of the mass flow rate,the cylinder champers pressure and piston dynamics equations and i am also using an equation for controlling the valve so i can control the system but i am not convinced with the results that i am getting so dose any one have any experience with simulation using the main equations that can help me or have any experience with the pneumatic system simulation please reply

ps If you want I can send you the model file and the equations