Our units have a check valve installed on the pure water line, at the membrane housings pure water outlet. It is built into the elbow fitting. If that valve does not work than the system will not function correctly. The ASOV will not shut the water off. Should it fail, the normal failure mode for this valve is to stick shut, not to stay open. In the rare event the check valve should fail, or your RO unit from another manufacturer did not come with a check valve, we can supply an in-line check valve so you will not have to disassemble the unit. Simply insert the check valve on the pure water line.