A drain line access device is provided for flushing HVACR drain lines. The device includes a main body having input and outlet connection ports on opposite ends for inline installation to a drain line, a flapper assembly sized and configured for seated placement in an interior cavity of the main body, and a removable top cover. The flapper assembly includes a flapper holder, a flapper, a flapper seat, and a spring hinge for rotatably urging the flapper into the normally raised position. An injection port is provided on the top cover for delivering a flow of pressurized gas or liquid for flushing out a clog in the drain line. In operation, delivery of pressurized gas or fluid into the drain line forces the flapper against the flapper seat, thereby sealing off one of the respective ends of the main body and directing the pressurized gas into the drain line.