Disclosed is an actuator including a cylinder volume having a first portion, and an actuator piston disc displaceable in the cylinder between inactive and active positions, an inlet channel between a pressure fluid inlet and the first portion of the cylinder volume, a first inlet valve body in the inlet channel, an outlet channel between the first portion of the cylinder volume and a pressure fluid outlet, and an outlet valve body in the outlet channel. The slave piston is displaceable in a bore between inactive and active positions. The first inlet valve body includes a seat valve body having an inactive position closing the inlet channel, the slave piston, in moving from inactive to active positions, rams the first inlet valve body, displacing it to an active position at which the inlet channel is open, the outlet valve body being connected to and jointly displaceable with the slave piston.