Ahh, finally.

I finally found the proof needed to show that toilet ballcocks (or at least the FluidMaster A400) work the way I described earlier in this thread. That is, by using the water pressure in the supply line acting on different areas on each side of a rubber diaphragm to allow or prevent flow of water into the toilet tank. Contrary to popular belief, the A400 does NOT use the buoyancy force of the float to stop the water flow, and so far as I know, no ballcock does. That was a contentious issue in this thread.

Readers are referred to Fluidmaster's own tech support page at this URL:

http://support.fluidmaster.com/reald...alve_works.pdf

The above PDF file explains how the diaphragm (which they call the "seal") is pushed up and down by water supply pressure acting on each side of it. And, most importantly, that the buoyancy force of the float is used only to raise and lower the "control stem" (which I called a "plug"), and whose movement effectively plugs and unplugs a hole in the diaphragm to release the pressure on the large area side of the diaphragm.

It's good to know how this kind of water closure system works because you find it all over the place in different products. Since the last time I posted, it's also come to my attention that Ondine bathtub diverter spouts also use a similar closure system to stop water flow out the tub spout, thereby forcing the water to go out the shower head.

Erness:
In the above PDF, you'll see that the rubber diaphragm is flexing in image #2. What I was trying to explain to you in an earlier post was that perhaps the reason why your ballcock wasn't closing was because the water pressure wasn't sufficient to cause the diaphragm to flex. That is, the rubber diaphragm is resilient and it takes a small but finite amount of force to cause it to flex. I was thinking you simply didn't have enough water presssure available to you to cause that rubber to flex. And, without the diaphragm flexing (as the result of movement) then the ballcock can't work properly. I expect what's happening is that the resilience of the rubber is causing the diaphragm to stay in the closed position, but not with sufficient force to close tightly so as to prevent water flow. So, what's probably happening is that water is merely "leaking" under the diaphragm into your tank because the force pushing down on the diaphragm to stop the flow isn't sufficient to overcome the force needed to flex the rubber.

If you ever need to replace the ballcock, try the kind I suggested earlier in this thread. I expect it'll work much better for you because it uses the hydrostatic pressure of the water in the tank to open and close the ballcock, not the water supply pressure. Your existing ballcock uses the water supply pressure, and that's problematic if you have almost none.

Clare_Snyder: The water valves in clothes and dish washers work much the same way, only they use a solenoid to pull a plug out of the diaphragm and a spring to push it back in.