I don't hear hissing and the system won't turn off.
Verify that the flow restrictor is in place in the black tubing. The flow restrictor produces that hissing sound as it provides back pressure, and if it is missing not only will the system not shut off, the RO system will cease producing pure water. All water will run out the drain line. In a pinch, install a 1/4" inline ball valve on the waste line and shut the valve enough to produce a waste ratio of 4 cups waste to 1 cup pure water. (4:1 is the standard ratio)