Everyone seems to be complimentary about these - but I have had a problem that I wonder if anyone could explain. I have one inside my house to isolate the outside tap. If I try to turn it off without opening the outside tap first it seems to jam. It is a screwdriver operated one. I wonder if some sort of seal is getting deformed by the back pressure. The first valve had to be replaced as I mangled the screwdriver slot. The second one (bought at the same time) is as bad.