19 and the wobble plate 17 are inclined relative to a radial plane and because the wobble plate is held against rotation rotation of the coupler 21 causes nutation of the wobble plate and this in turn causes reciprocation of each of the positive displacement pumping members 101 on the intake stroke of the pumping member 101 shown in fig1 this pumping member moves downwardly as viewed in fig1 to draw water from the inlet 131 into the pumping chamber 113 on the discharge stroke the pumping member 101 moves upwardly as viewed in fig1 to force water from the pumping chamber 113 through the outlet opening 121 and the outlet valve 139 to the outlet 135 as described more fully in u s pat no 4 153 391 referred to above each of the three pumping members 101 is repeatedly moved through intake and discharge strokes in this same manner with each of the pumping members being 120 u00b0 out of phase with the others as pressure at the outlet 135 increases pressure in the pumping chambers 113 on the discharge stroke also increases this back pressure resists the discharge stroke of the pumping members 101 and this tends to pivot the coupler 21 clockwise as permitted by the oversized driving portion 39 of the bore 37 from the position shown in fig1 and 2 to the position shown in fig3 against the biasing action of the resilient seal member 25 in fig1 and 2 the part cylindrical surface 68 engages the shaft 23 to define a maximum wobble plate angle the seal member 25 is preloaded between.