Access system for forming and enlarging a percutaneous penetration are disclosed. The apparatus includes a radially expandable dilation assembly having a radially expandable sleeve body and a handle portion operatively coupled to the proximal end of the radially expandable sleeve body. The access system further includes an expansion assembly having an expansion member configured and adapted to radially expand the sleeve body upon insertion therein of the expandable member. The handle portion including a seal disposed across an aperture formed therein for creating a fluid-tight seal around a proximal end of the expansion member. The handle portion including a valve stem operatively and fluidly coupled thereto for delivering a fluid into the aperture of the handle portion. The expansion member including means for reducing the force required to slidably insert the expansion member into the sleeve body of the dilation assembly.