Discharge pipes are usually PVC, stainless steel or fiberglass. The bell, similar to an inverted barrel, covers the discharge pipe so that air is trapped in the system. Usually, the bell is made of stainless steel, fiberglass or plastic. The vent hole in the bell allows the inside and outside air pressure to equalize after each flush. This is necessary to ensure reliable operation of the siphon.
Air is released from under the bell to activate the siphon and complete a flush.
Air-release devices include manually operated valves, flexible hoses or automatic trigger tubes. The trap is a reservoir under the tank that holds air in the system. The trap also provides a flow avenue for the flush water from the discharge pipe to the gutter. As the tank fills, water covers the vent hole in the bell and a specific volume of air is trapped under the bell and in the discharge pipe. After the vent hole is covered, the water level under the bell rises at a slower rate than the water level in the tank.