an apparatus for actuating a pressure delivery system of a fluid sampler . the apparatus includes a housing having a longitudinal passageway and defining first and second chambers . a piston is disposed within the longitudinal passageway between the first and second chambers . a valving assembly is disposed within the longitudinal passageway . the valving assembly is operable to selectively prevent communication of pressure from a pressure source of the fluid sampler to the second chamber . the valving assembly is actuated responsive to an increase in pressure in the first chamber which longitudinally displaces the piston toward the valving assembly until at least a portion of the piston contacts the valving assembly , thereby releasing pressure from the pressure source into the second chamber and longitudinally displacing the piston away from the valving assembly .