Since the USA and UK are tow countries separated by a common language, I asume your term cistern is the same as our flush tank. If so get back and we will compare notes. In our country cistern would refrer to a water storage tank, such as the one in many European countries, for water supply to the entire house except for drinking water.: Ive been trying to understand the operation of a toilet cistern in the usa, its obviously different to uk systems, would any of you be kind enough to explain?