This paper deals with the estimation of physical parameters of a pneumatic servo positioning system. It consists of two servodistributors and an actuator. This work concerns not only the identification of the mass, viscous and Coulomb non symmetric frictions parameters of the mechanical part but also the polytropic coefficient of the gas considered as a perfect one. At first the mass flow rate characteristic of the servodistributor is determined indirectly by an approximation using several polynomial functions. Next, a dynamic model which is linear in relation to a set of dynamic parameters is proposed. The dynamic parameters are estimated using the weighted least squares solution of an over determined linear system obtained from the sampling of the dynamic model along a closed loop tracking trajectory. An experimental study exhibits good identification results.