If your level is dropping your ball valve is probably siezed.

Usually there is so little flow that it locks up in the closed position
after a year or so and then the header tank slowly empties.

Try waggling the ball valve to confirm that water comes in when you press
the ball down and then stops when you release it (wouldn't want a sticky
valve to stick open).

"4square" wrote in message
...
In an open-vented system, anyone know how much water is used, by
evaporation from the tank, if any? level in my tank dropping slightly,
maybe slight lk somewhere I suspect.