Abstract: A cylindrical cap having a closed end except for a single small opening is internally threaded and arranged to receive a resilient washer and resilient disc overlying the washer. The disc has a small single opening. A coupling tube is threaded into the cap to hold the disc against the annular washer so that a small area is defined within the annular shoulder defined by the washer. The coupling tube includes a stem at its other end for connection to a lateral water line in an irrigation system. The arrangement is such that when water pressure is below a predetermined operating pressure, the water can pass through the opening in the disc and out the small opening in the end cap to purge the line.