We introduce passive mobile robots consisting of various wheels; omnidirectional wheels or one-wheel casters, which are controlled by servo brakes. These robots can realize several functions, such as path tracking, gravity compensation on a slope, and collision avoidance function, by controlling only the servo brakes. They employ passive dynamics with respect to the force applied by a human. In this paper, we derive and analyze a feasible braking force/moment that is necessary for controlling the robot with servo brakes. We reveal the advantages and disadvantages of the feasible braking force/moment of each robot developed by us. In addition, we propose a new passive type double wheel caster unit, called PDC, on the basis of the analysis. We develop a prototype PDC called PDC-P1. Finally, we handle a long object by two PDC-P1s actually, and realize the path tracking function as example to confirm its validity.