Reverse Osmosis would not shut off after tank was full.

Remove the reverse osmosis unit fron under the sink and disconnected the hosed needed to manuever it to remove the plastic lines to the auto shut-off valve. Exchanged the valve but unfortunately this was not the problem. I believe that the above sink water dispenser is the issue. I have not had a chance to order a new water dispenser,but that is the next step. The original complaint was that the unit does not shut down after the storage tank is full and there is a persitant distinct intermittant shreech sound coming form the unit when this problem first presented itself. This led me to think it was the autoshut-off valve.