Abstract: The mechanism for flush system to control water-draining of tanks at atmospheric pressure comprises a shutter with a resilient valve freely floating in the water of the tank, a seat for said shutter, an air-containing bell topping the shutter and provided at the upper portion thereof with an upper valve and having inner size which is larger than outer size of the resilient valve, the bell and the resilient valve being provided to enable - when the upper valve is closed, thus preventing the air inside to go out - the resilient valve to apply on the seat of the shutter thus ensuring closing of the tank which is then supplied with water while compressing the air which is in the bell, and when the upper valve is open, part of the air in the bell escapes therefrom and is replaced by some water, which releases the resilient valve from the seat of the shutter and consequently provides a draining of the tank.