Abstract: A continuously operable piston pump for conveying of viscous materials, such as mortar mixes or the like comprises first and second chambers separated from a pressure source, such as a piston cylinder arrangement, by respective flexible wall means. The chambers are connected in series with each other. The pressure source is controlled by a further piston cylinder arrangement including a control valve at each end of the control cylinder. The control valves are operated by a flip-flop means which in turn is actuated by means directly coupling the flip-flop means to the piston rod of the control piston cylinder arrangement. The pressure piston cylinder arrangement and the control piston cylinder arrangement are constructed to have a common axis, and thus also a common piston rod located in said axis. The control valves are connected to said common piston rod.