Abstract: A quick-acting closure unit for temporarily sealing the open end of a line used to convey a fluid under pressure. A cylindrical sleeve having a lower section that is tapered inwardly is passed over the open end of the line which contains a collapsible jaw means slidably mounted within the lower section that encircles the line. A cup-like member, which is also slidably mounted in the sleeve, is passed over the open end of the line and contains a sealing assembly that, when compressed, produces a fluid-tight seal between the cup and the line. A camming lever is arranged to act between the sleeve and the cup to force the cup downwardly into driving contact with the jaw means. The downward movement of the cup initially causes the seal to be compressed thereby closing off the line and then forces the jaws into holding contact with the line to secure the unit in a sealing posture thereon.