mouth 82 to the outer diameter of backface 81 and are spaced from last machined surface 83 by a gap each vane 85 curves in a manner to create a differential pressure between the seal cavity surrounding gland 77 and the exterior with the pressure being greater in the seal cavity 77 this pressure tends to cause drilling fluid to circulate out of seal gland 77 vanes 85 cause the volume of the seal cavity which includes seal gland 77 and the gap between backface 85 and last machined surface 83 to change at any selected point on last machined surface 83 due to rotation of cone 75 other configurations to provide a variable volume for the seal cavity are feasible for example the cone mouth could be noncircular but rotate about the bearing pin axis the cone mouth could be oblong or have one or more recess portions within it that result in a noncircular inner diameter at the entrance portion of the cone cavity these recess portions could be formed at different radial distances from the seal assembly the change in seal cavity volume need not occur gradually as in the first embodiment there could be more than one maximum width and minimum width portions as provided by the vanes of the third embodiment rather than gaps between the cone and bearing pin differing in radial dimension recess portions could extend axially from the backface of the cutter at different distances to change the volume of the seal cavity at different points around the circumference of the cavity mouth in that instance.