{HEAD ~ A seal ring} located in a seal ring groove of a valve member of a rotary valve assembly has {TAIL ~ an outer seal surface} in sealing engagement with a surrounding valve housing and is adapted to isolate pressure passages communicating with the valve assembly regardless of the direction of the pressure differential across the seal ring, the seal ring being maintained in sealing engagement with one wall of the seal ring groove and with a surrounding wall of the valve housing by an underseal expander ring that assures continuous sealing action of the seal ring 
has
{ HEAD ~ A seal ring } located in a seal ring groove of a valve member of a rotary valve assembly has { TAIL ~ an outer seal surface } in sealing engagement with a surrounding valve housing and is adapted to isolate pressure passages communicating with the valve assembly regardless of the direction of the pressure differential across the seal ring , the seal ring being maintained in sealing engagement with one wall of the seal ring groove and with a surrounding wall of the valve housing by an underseal expander ring that assures continuous sealing action of the seal ring
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