The valve block is sealed into its mounting position by a number of sealing rings (30) which sit between the block and the housing, or the bore into which the block is inserted. The sealing rings have a smaller outer dimension than thrust rings (32) spaced between the rings and which position the rings. The thrust sleeves have connections to the separate valve ducts and provide an axial thrust as the block is tightened into its mountings causing the sealing rings to expand and effect the seal.
