A fluid pressure actuator for a turbocharger system's turbine bypass valve comprises a piston slidable in a cylinder so as to define a chamber containing a compression spring assembly operable to exert a spring force on the piston. The cylinder can be selectively subjected to a vacuum or pressure for exerting a fluid pressure force on the piston in a direction opposite from the spring force. The spring assembly comprises a first spring arranged to be compressed by the piston throughout a first range of motion of the piston in a compression direction, and at least a second spring arranged to be compressed by the piston throughout a second range of motion that is smaller than and co-terminal with the first range of motion but to be uncompressed during an initial part of the first range of motion of the piston in the compression direction.