be provided on the hammer drill 10 a reduction valve 20 operable by a handle not shown and a slide valve 21 the slide valve 21 may for example be arranged for convenient manipulation from the grasping handle 19 by means of a trigger not shown mounted thereon the hammer drill 10 has a conventional throttle valve 22 to which valve pressure fluid is conducted by means of a fluid supply line 23 according to fig1 in the diagrammatically illustrated position of the slide valve 21 there is established a communication 24 from the throttle valve 22 via the reduction valve 20 the slide valve 21 and the hinge connection 11 to the upper working chamber 14 by manipulation of the handle of the reduction valve 20 the working chamber 14 may be supplied with fluid at a pressure reduced at will in order to provide for efficient feeding of the drilling tool forwardly by extension of the pusher leg 12 simultaneously therewith a communication 25 via the hinge connection 11 and the slide valve 21 connects the lower working chamber 15 to atmosphere in order to contract the pusher leg the pressure fluid supply to the pusher leg preferably may be reversibly arranged by means of the slide valve 21 which in another position not shown vents the upper working chamber 14 via the communication 24 and connects a direct passage 26 from the throttle valve to the communication 25 i e to the lower working chamber 15 of the pusher leg for contracting the leg in connection with the general arrangement of the drilling tool and the pusher leg described above.