This paper presents a model-free robust/adaptive control scheme for tracking control of electro-hydraulic servo systems using output feedback only. The Taylor expansion theory is used to enable the proposed controller to identify and feedback linearize the dynamics of a system on-line, and to compensate parameter variation and various disturbances in real time. As a result, the closed-loop system of the complicated electro-hydraulic system behaves as a simple linear dominated system plus a small uncertainty residual. By properly designing the adaptive law of the robust gain, asymptotic stability and satisfactory performance of the closed-loop system can be achieved. The details of controller design and stability analysis are presented in the paper. The controller has been implemented on a plastic injection molding machine to control the position of the mold-supporting movable platen. The results from experiments indicate that the proposed controller performs much better than the conventional PID feedback controller.