This work investigates the model based control of an affordable pressure regulator consisting of a pressure sensor and two identical flow restrictors actuated by DC servo motors. The system model is constructed considering the pressure dynamics and the lag due to the servo motors. The main contribution is a new model-based passivity-based controller for pressure regulation which employs a nonlinear observer to estimate the uncertain model parameters and to compensate for air leaks. The effectiveness of the controller is demonstrated with numerical simulations and experiments, and the prototype is compared to a commercial digital pressure regulator.