Apparatus for sealing a puncture includes an elongate occlusion member having a balloon attached to distal ends of telescoping inner and outer members. A housing on the proximal end of the outer member includes a piston coupled to the inner member and slidable within a chamber communicating with a fluid reservoir. A switch on the housing is actuated to direct fluid from the reservoir through the outer member into the balloon to expand the balloon and into the chamber to move the piston and pull the inner member, shortening the balloon as it expands. During use, the distal end of the occlusion member is introduced into a puncture communicating with a vessel until the collapsed balloon is disposed within the vessel. The balloon is expanded, and a tensioner is connected to the housing to apply a proximal force holding the balloon against the vessel wall to seal the puncture.