how much water should remain?

It is not uncommon for water to remain in the cistern it can depend on type of syphon and flush mechanism you have. Some flushes give a half or a full flush depending on how they are operated. Some syphons are adjustable to how much water they let out in a single flush. eg if your flush is set to say 10lt and your tank holds 12lt then 2lt will remain etc. It may be that the level is a little too high meaning water is not all used. Most cisterns will have a fill level marked somewhere on the inside. Try adjusting your filler to this level.
However I would say that as long as the flush is good enough to clear the bowl then dont worry.

Robin S
September 2008