Abstract: A stapler wherein the piston which drives home discrete fasteners is received in a cylinder having an open end overlapped by the valving element of a main valve. The valving element is permanently biased to sealing position by a plastic diaphragm spring and is normally held in such position by pressurized gaseous fluid which is supplied by way of a passage containing a second valve and a reciprocable second valving element close to a chamber at the outer side of the valving element of the main valve. The second valve seals the passage from a source of pressurized fluid and simultaneously establishes communication between the passage and the atmosphere in response to depression of a trigger. This results in a reduction of pressure in the passage whereby the pressurized fluid in the chamber shifts the second valving element to a position in which the chamber is free to communicate with the atmosphere at a location close to the chamber.