an improved radial , pressure - balancing arrangement for fluid - operated devices of the geroter type wherein an externally - toothed star member is eccentrically disposed within an internally - toothed ring member to form a plurality of expanding and contracting volume chambers by tooth interaction . the ring member has a plurality of pockets receiving a like number of rollers rotatable therein which define the teeth of the ring member . particularly configured recesses are located at predetermined positions within each pocket . the recesses are effective to provide a predetermined pressure pattern behind a selective roller at any particular time during the operation cycle . the pressure pattern thus formed biases the selective roller against a mating star tooth to seal volume chambers at high pressure from those at low pressure .