83 and moves the member 82 into a position which blocks the passage 74 as shown in fig3 b the valve 80 further includes a spring 86 which insures that the passage 74 is open when the disc 10 is not rotating an o ring 90 seals the passage 74 when the valve 80 is closed it will be understood by those skilled in the art that the centrifugally operated valve 80 shown in fig3 a and 3b is but one of many possible embodiments of a centrifugally operated valve for example a positive snap action may be desired upon closing of the valve the counterweight 83 is shown to clearly illustrate the operation of the valve 80 however the member 82 can have any shape which is adapted for movement by centrifugal force the spring 86 can be eliminated when the disc 10 is loaded and unloaded in a horizontal position and the force of gravity is sufficient to open the valve means for controlling the operation of the centrifugally operated valve 80 is shown in fig1 and 2 a pressure sensor 94 such as a diaphragm type is positioned in the implant chamber 22 so as to sense the pressure in the vicinity of the rotating disc 10 the output of the pressure sensor 94 is coupled to a level detector 96 which provides an output signal.