Continuously operating piston pump

A continuously operable piston pump for conveying viscous materials comprising first and second conveying chambers, each of said chambers having at least one flexible wall means partly defining the respective chamber, conduit means operatively connecting the first and second conveying chambers in series, differentially operable cylinder piston means with a fluid therein for operating said flexible wall means which sealingly separate the fluid from the viscous material, inlet and outlet conduit means communicating respectively with said first and second conveying chambers, nonreturn valve means disposed at said inlet conduit means, and at said first mentioned conduit means, and means arranged for controlling said differentially operable means, said control means including flip-flop means for reversing said cylinder piston means, said control means further comprising control cylinder piston means, a common axis on which said differentially operable cylinder piston means and said control cylinder piston means are arranged, common piston rod means extending along said common axis and out of said control cylinder piston means, and means connecting said flip-flop means to said common piston rod means.