A sealing arrangement for a rotating coolant union for preventing leakage forward between the outer surface of a seal carrier of a floating seal assembly of the coolant union and the inner wall of the housing, includes an O-ring seal and an annular back-up ring. The O-ring is mounted on the seal carrier and extends into an annular groove defined in the inner surface of the housing, with the width of the groove being greater than the outer diameter of the O-ring seal to permit the O-ring seal to be moved within groove in response to axial sliding movement of the seal carrier. The back-up ring encircles the support member and includes a radially projecting portion that extends into the groove and defines a slanting surface for a corner of the groove, the slanting surface being engaged by the O-ring seal during relative axial movement between the seal carrier and housing for limiting movement of the seal within the groove.