A flush valve assembly for use in a toilet tank that has a drain opening, a valve seat at the drain opening, and a valve member movable relative to the valve seat, together acting as a flush valve. The valve member has an internal chamber for containing water to reduce buoyancy of the valve member in water, so that the valve member may descend through water to reach the valve seat for sealing engagement with the valve seat. The valve member has an inlet for admitting water into the chamber, and an outlet for discharging water from the chamber. A toilet flush system includes the toilet tank, a tank refill mechanism for refilling the toilet tank with water, the flush valve assembly in the toilet tank, and an operating unit for operating the flush valve assembly.