Abstract: A pump, characterized by flexible tubes which are cyclically expanded and contracted to cause fluid to be pumped, comprises a pair of liquid filled cylinders each containing a flexible tube connected to inlet and outlet valves, a liquid filled control cylinder communicating at its opposite ends with respective ones of said pump cylinders, and a piston reciprocable in said control cylinder for alternately transferring fluid to and from said pump cylinders for alternately and cyclically expanding and contracting said tubes to cause fluid to be pumped in a double acting pump action. The pump is especially well suited for pumping abrasive and corrosive slurries, heavily pigmented and/or metal filled coating materials and the like. The pump is characterized by improved piston means, improved sealing and retaining means for both the piston and the tubes, and means for detecting leakage across the piston, whereby to increase pump reliability and longevity of service, and to monitor internal pump conditions.