The present invention provides an improved design for a mechanical pump. The pump is a dual chamber design typically including a pair of expandable bellows driven in a reciprocating fashion by a supply of pressurized air. The use of expandable bellows reduces wear on the pump thereby lengthening the required maintenance intervals in comparison with dual chamber diaphragm pumps previously known. Additionally, the pump is easily disassembled for inspection, cleaning or maintenance. Furthermore, the high strength of the pump's design makes it possible to manufacture the pump entirely of corrosion resistant materials.