Ahh, finally. I finally got the proof I needed to show that toilet
ballcocks (at least the FluidMaster A400) use the water pressure in the
supply line to open and close, and not the buoyancy force of the float,
which was a contentious issue in this thread.
Readers are referred to Fluidmaster's own tech support page here
The above PDF file explains exactly how the diaphragm (which they call
the "seal") is pushed up and down by water pressure acting on it.
Clare_Snyder: The water valves in clothes and dish washers operate
exactly the same way, it's just that they use an electromagnet to pull a
plug out of a hole in the diaphragm and a spring to push it back in.
But, it's still the _water_pressure_ that pushes the diaphragm off the
seat and water pressure that pushes it back onto the seat, not the
electromagnet. The electromagnet merely pulls the plug out to release
the water pressure on the large area side to cause the valve to open and
the spring just pushes that plug back in to cause the valve to close.