Understanding how toilets work so that we can adjust them

When the cistern is full, the flush valve seal sits across the bottom opening which would otherwise empty the water down to the toilet below. The cistern seal is held in place by the pressure of the water above. Meanwhile, the float on top of the seal is trying to lift the seal and let the water escape. The downward water pressure is greater than the lifting force of the float, so the seal stays in place and the water remains in the cistern. (In fact, the float is normally underneath the seal. I have just drawn it this way as it gives a better demonstration of the principle involved).

(Inlet valve not shown) 

Once the water has emptied into the toilet bowl below there is no water left to lift the float. Gravity takes over and the seal once more falls shut across the opening.

The inlet valve now begins filling the cistern. The incoming water pressure holds the seal shut over the opening. As the water rises the float will try to lift the seal up. However, the water pressure bearing down is strong enough to overcome the float’s lift, so the seal remains closed.

(Inlet valve not shown)

 The inlet valve will continue to fill the cistern until the incoming water reaches the correct level and shuts off. The cistern is then “loaded” with water until the next flush.