To prevent the run-away of a vehicle by mounting a self-contained valve to resupply operating oil leaked from a motor or an oil pump to main pipe passages for a closed circuit, even if the motor for the vehicle is stopped on a slope or the vehicle is moved down on the slope with a self-weight, a wheel is rotated and a hydraulic motor is rotated via the wheels for pumping operation.

A fluid driver has a main pump 1 and a charge pump 2 driven by a motor, a hydraulic motor 3 connected from the main pump to a closed circuit formed with main pipe passages, check valves 7a, 7b mounted on a charge passage communicated between the main pipe passages to supply operating oil from the charge pump to the main pipe passages, a low pressure relief valve 6 provided in the charge passage to escape charge pressure into a case 13, a high pressure relief valve 8 to restrict high pressure in the main pipe passages, neutral valves 9a, 9b to escape operating oil into the case in small pressure areas in the main pipe passages and an unload valve 10 connected between the main pipe passages to unload high pressure in the main pipe passages into low pressure. A self-contained valve 17 is mounted thereon to allow only the flow of operating oil from the case 13 to the charge passage 11.