Abstract: A flush valve assembly for use in the water storage tank of a toilet, the closing speed of which can be controlled and adjusted to seal the storage tank as soon as the trap in the toilet bowl has been refilled, thereby preventing water loss from an overfilled trap. A valve cup having a float is rotatably mounted within a housing and has an elongated horizontal slot or the like near the lower edge. The valve cup contains water when the storage tank is full, and upon initiation of a flush cycle, the float engages an abutment which is secured to the valve shaft and retards the rate of descent of the valve shaft by floating downwardly on the water in the valve cup as this water drains from the horizontal slot. The valve cup can be rotated to variably restrict the slot, thereby increasing or decreasing the rate of descent of the valve shaft to seal the tank as soon as the trap has been refilled.