A servo system includes pressurized fluid, and one or more fluid control devices (FCD) such as a valve or actuator, with at least one FCD having an element with a variable property that varies in response to a stimulus. The FCD controls a flow rate or pressure transmission of the fluid within the servo system. The element may include an active return spring having a variable stiffness. The servo system may operate as an interlock system for a transmission. A method for moving an output load using a servo system includes providing a first FCD with an active element, connecting an output load to a second FCD, activating the active element with a stimulus to vary a property of the active element, transmitting a force from the pressurized fluid to the second FCD, and moving the output load from a first position to a second position in response to the force.