manner illustrated wherein the blind bore 16 communicates with the hydraulic fluid reservoir 6 through a passage 18 and with the working chamber 9 through a passage 19 as is evident this arrangement and design of the relief valve 15 thus enables the working chamber 9 to communicate with the hydraulic fluid reservoir 6 through the passage 19 18 and the pressure of the working chamber 9 to be reduced in case an inadmissibly high pressure is built up within the working chamber 9 during the delivery stroke of the diaphragm 10 in corresponding manner a relief valve 21 is disposed within another blind bore 20 of the cylinder body 2 to provide communication between the working chamber 9 and the hydraulic fluid reservoir 6 for the purpose of affording protection from subpressure when the diaphragm 10 is in abutment against the working chamber wall during the diaphragm suction stroke for this purpose the blind bore 20 communicates with the working chamber 9 through a passage 22 and with the hydraulic fluid reservoir through a passage 23 whilst the relief valve 21 includes in the manner illustrated a spring loaded valve ball member 25 which abuts against the lower side of the bottom of an insert member 24 and which separates from the bottom of the insert member 24 when a certain preset subpressure is attained thus.