Abstract: A hydraulic drive system for reversibly operating a device such as a wheelchair ramp or other type of mobility access device is provided. The hydraulic drive system includes a bi-directional power unit that is in fluid communication with a cylinder for deploying and stowing the ramp. The system further includes valves disposed between the cylinder and pump, which may be spring biased shuttle-type valves. In one embodiment, the valves are normally biased so that the cylinder and ramp may move freely such that the ramp may be manually operated during a loss of electrical power to the power unit.