For any newbies in here who want to learn how toilet ballcocks work, this web page from FluidMaster tech support explains in step-by-step detail how the rubber diaphragm in a FluidMaster 400A toilet fill valve allows water flow into the tank, and shuts off the flow when the tank is full.

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

Unlike the mechanical systems typically used on faucets, the closure system used on the FluidMaster A400 toilet fill valve uses the water supply pressure to allow flow into, and stop flow into, the toilet tank. This is a very simple and reliable way to allow and stop water flow which is likely to become more common in the future because of it's simplicity and reliability.

It's good to understand how this water shut-off system works because you find the same kind of closure in lots of other places. Clothes and dish washers use much the same closure system to fill the appliance with water prior to each washing cycle. Similarily, some toilet diverter spouts use this same closure system to prevent water flow out the tub spout when the shower is supposed to be on. And, of course, all toilet fill valves use a variation of this same system to fill the toilet tank after each flush.

It's just plain good stuff to know.