[mike31154]

Thanks for that info re the pressure tank. I don't have a pressure tank on my system and the ASOV has never worked since day 1 so I've always done what lastlight does, keep an eye on the system and manually shut it down. Took it apart to have a look and still no joy when I reassembled it. Pretty simple really, just a diaphragm that should close the openings on one side with sufficient backpressure, so apparently not much that should go wrong. But due to my own experience with it, I would never trust that thing on it's own without being monitored. I think I may have got it to work a couple of times by cycling the membrane flush valve, but certainly not reliably. Maybe I'll try adding a pressure tank for RO at some point to see if that helps.