a downwardly facing shoulder 70 and the drive member 62 has an upwardly facing shoulder 72 confronting the downwardly facing shoulder 70 the shoulders 70 and 72 have complementary cam surfaces which control the vertical height of the valve stem 34 as will now be described with reference to fig6 and 8 the valve stem 34 is molded in one piece and has a lower end which comprises a cylindrical body of a size to close the port in the valve seal 35 and a downwardly extending notch or recess 74 of increasing dimension as is apparent the valve stem 34 when lowered as shown in fig6 fully closes the port in the valve seal 35 and opens the port by increasing degrees when the valve stem 34 is raised the upper end of the valve stem 34 comprises an integral cam follower 76 that extends into the space between the shoulders 70 and 72 an integral pair of arms 78 that engage the outer surface of the drive member 62 to prevent the valve stem 34 from rotating and an integral triangular rear portion 80 that engages between the cylinder portions of the pump housing 81 so that the head of the valve stem 34 is always held in a position in which the cam follower 76 extends between the shoulders 70 and 72 fig4 and 5 show how rotation of the temperature control.