A modular apparatus for controlling flow of pressurized hydraulic fluid to and from opening and closing hydraulic actuator cylinders of oil and gas well blow-out preventers (BOP's), utilizes novel rotary hydraulic valve/actuator assemblies, each of which uses a pair of integral double-acting pneumatic actuator cylinders that drive a rack gear coupled to a spur gear located inside the actuator housing which is fixed to a valve rotor shaft that protrudes forward from the valve housing and protrudes through an outer wall of the housing and has a manually operable handle attached thereto, thus enabling multiple valve/actuator assemblies to be mounted in a close side-by-side arrangement to an hydraulic manifold. An air control manifold panel for remotely energizing the pneumatic actuator cylinders includes opening and closing push button control valves on an air manifold connected through air hoses to the actuator cylinders.