Abstract: A pressure fluid apparatus includes first and second parts which are adapted to be secured together. A first sealing surface of the first part forms a seal with a corresponding second sealing surface of the second part. The first sealing surface includes a spigot, and the second sealing surface includes a corresponding recess adapted to receive the spigot. The inside corner where the base of the spigot meets the first sealing surface is radiussed, and the outside corner where the mouth of the recess meets the second sealing surface is correspondingly truncated, so as to accommodate the radius. On either the radiussed inside corner or the truncated outside corner, an integral projection is provided which is deformable upon assembly of the parts, to block leakage of fluid along the path defined between the two corners.