Check the listing for details. Reverse Osmosis System Automatic Shut Off Valve (ASO). Condition: New. Listed at 10.99 USD. Automatic Shut Off Valve (ASO) For Reverse Osmosis SystemsOur ¼” Quick Connect ASO allows your system to turn off the water supply when the tank pressure reaches approximately 65% of the supply water pressure. When you have a tank full of clean water and the waste water didn’t shut off this might be an indication to change your ASO Valve. Other indications are: ASO rupture or burst – which can cause pure water to mix with tap water.ASO leak and water can’t get through all the valve causing water channels to plugRemark: Continual drain problem can be caused by different factors, including automatic shut-off valve, check valve, flow restrictor, or tank pressure. Replacing the ASO may not help if the problem is caused by other factors.