Recently, a passivity-based control has been investigated a lot because robots have been used frequently for outdoor applications. However, since conventional hydraulic robot systems which use flow-control servo-valves are not an inherent passive system, such developed passive control algorithms cannot be applied for conventional hydraulic robot systems easily. Therefore, this paper proposes a control method that makes the conventional hydraulic robot systems passive. More specifically, a pressure feedback control of electro-hydraulic servo-valves is introduced with a simple low-pass filter. As a preliminary work, simulation has been performed to verify proposed control method and shows that the modified hydraulic robot system is passive.