Abstract: A valve structure comprises an elongated housing having a passage extending therethrough, and a control member mounted in the passage for rectilinear movement with respect to the housing. The housing has a plurality of spaced valve ports extending therethrough in communication with the housing passage. A plurality of sealing means are non-uniformly spaced along the length of the control member for movement therewith, with each of the sealing means sealingly engaging adjacent wall surfaces of the housing passage. The sealing means in one position of the control member define a given flow circuit among the valve ports, and in another position of the control member its sealing means define a different flow circuit among the valve ports.