An apparatus for sealing a puncture communicating with a blood vessel includes an inner member slidable within an outer member, and a balloon coupled to distal ends of the inner and outer members. A proximal end of the outer member includes a port for delivering fluid into the balloon, and a cylinder that communicates with the port. A piston coupled to the inner member is slidable and biased to move distally within the cylinder. The apparatus is introduced into the puncture until the collapsed balloon is disposed within the vessel. Fluid is introduced into the port, moving the inner member proximally to shorten the balloon as it expands. The balloon is withdrawn to seal the puncture, and a hydrogel is introduced into the puncture. Fluid is removed via the port, moving the inner member distally to extend the balloon as it collapses, and the apparatus is withdrawn.