I believe you need one of these:
http://www.aquariumwaterfilters.com/...p&Itemid=6

The automatic shut-off valve will regulate water flow for the RO. When the back pressure or float drops, the valve is activated and water is allowed to flow through the water filter to the storage tank or float valve. When the pressure tank or storage tank is full, the pressure in the line signals the valve to shut off water flow through the filter.