: If it has the tank entirely above the bowl, then all the flush pressure is provided by the water in the tank. If it is a low "non overflow" type then about half the water for the flush comes from the inlet pipe, but is only slightly dependent on the inlet pressure.

: : Does a gravity flush toilet need a certain amount of
: : inlet water pressure {above that reqiured to fill the tank} to operate properly.

Pressure assist toilets work best at 20 PSI or more. Most gravity toilets can get by with less. As hj pointed out, some of the older style low-tank toilets used the fill water to perform some of the flush action. Terry