A well sealing tool may include a hydraulic setting mechanism wherein a setting chamber is isolated after setting a sealing element in engagement with a wellbore. In one example, a setting mechanism includes a setting chamber housing positionable about a mandrel to define at least a portion of a setting chamber between the mandrel and the setting chamber housing. A setting port fluidically couples a through bore of the mandrel with the setting chamber. A valve element is biased toward a closed position within the setting port. A guide sleeve is disposed about the mandrel in a first position that props the valve element to an open position. The guide sleeve is moveable to a second position in response to a threshold pressure applied to the setting chamber to release the valve element to the closed position.