23 is mounted in the apex of the dividing member at its position of contact with the shaft 13 so that the shaft can rotate while maintaining effectively a seal to prevent the passage of air between the shaft and the dividing member the dividing member 21 acts as a manifold for communication of air from a pair of inlets to a pair of outlets for cooperating in the pumping action specifically as best shown in fig1 a first inlet duct 24 extends through the housing 10 to an inlet port 25 on one face of the dividing member a second inlet duct 26 extends through the housing 10 into the dividing member 21 and terminates at a second inlet port 27 provided in the second face of the dividing member which is opposed to the face containing the port 25 similarly a first outlet duct 28 extends through the dividing member and terminates at an outlet port 29 on the first face of the dividing member a second outlet duct 30 extends to the dividing member and terminates at a second outlet port 31 on the second or opposed face of the dividing member mounted on the shaft 13 is a vane 32 which lies in an axial plane of the shaft and follows substantially the shape of the hollow chamber so that it can rotate with the shaft in a sweeping action around the hollow chamber around the periphery of the vane is a sealing member 33 shown in most detail in fig3 the sealing member comprises a first support section 34 mounted upon an outside edge of the vane in the form of a channel which engages the edge and clamps around the edge a sweeping or sliding or sealing flange portion.