A piston device having an inlet through which the piston device can receive liquid and an outlet through which the piston device can inject pressurize liquid. A main body (28) of the piston device defines an inner chamber and an upstream duct placing the inner chamber in fluidic communication with the inlet. A valve having a valve plate, which is movable along a plate actuation path, between at least an opened position of the plate and a closed position of the plate.The plate actuation path extending at least in part inside the upstream duct.