This paper presents a proposal of new hydraulic circuit, designated as a modular hydraulic servo booster (MHSB), aimed at the realization of modular hydraulic robots. The modular robots, however, have important shortcomings compared to non-modular robots, such as separate power sources and power imbalance between axes when applied to serially configured robots. To mitigate those difficulties, we take advantage of pressure boost and flow summing by multiple servo pumps and switching valves, which are connected through the hydraulic rails shared between the circuits. Coordinated control of the pump and valve greatly improves energy efficiency over conventional servo valve systems. After presenting realization of the circuit and possible operating modes, the control method for each mode is explained. The experimentally obtained results for position control of a two-link serial manipulator validate the proposed method, especially by the shared boost mode, where small pumps work together to have high torque and speed of the proximal joint.