Leaky dual siphon flush system after chaging diaphragm?

Recently I changed the diaphragm of a siphon type flush system. Now I am able to get the dual flush.

For half flush the water from the tank continues to flow after the flushing. Ideally after the half flush water should stop flowing into the pan.

I have tried to fiddle with the water level and change the rate of water that enters into the tank. Found that water keep on flowing after the half flush.

Any one with any pointers how to resolve this issue. Thank you.