AbstractAccurate system modelling is becoming more and more important in the analysis and synthesis of hydraulic systems. Pumps with constant-pressure regulators are becoming increasingly common in hydraulic systems. Although these pumps often play an important part in the dynamic behaviour of a particular system, their dynamic properties are often unknown.In this paper it is shown that a simple two-parameter linear model is usually sufficient to describe the dynamic behaviour of a pump with a constant-pressure regulator, provided the disturbances are not too large. For a particular pump it is shown how these parameters are related to the system variables–pressure and flow. These two parameters are the pump inductance (inertance) and a characteristic break frequency for the pump.Due to the many non-linearities of the regulator, large disturbances will require simulation of the non-linear equations. The pump is supposed to be connected to a system which can be described with concentrated parameters, and the response to various disturbances is investigated. The simulation program used, HOPSAN, has the advantage of including a routine for FFT-analysis, which provides a possibility of determining open-loop and closed-loop response of the control system directly from a simulated response of the system with step input.The results of linear models and simulation of nonlinear models are compared with experiments. The agreement is good.