When a faucet in the house is used and then turned off, the toilet on the first floor runs briefly. Can be a big surprise to someone when the toilet is in use. How can I fix this?

 

Hunter:

The answer to your question lies in the fact that toilet fill valves no longer use the buoyancy force of the float to pinch off the water pressure. Nowadays, every toilet fill valve that I know of uses the same principle as a washing machine to shut off the water when the tank is full. That is, the water pressure itself is used to hold a diaphragm tightly over the end of the pipe where the water comes in.

The diaphragm has small holes in it that allow the pressure to equalize on both sides of this diaphragm. Pressure equalization happens almost instantaneously because the volume on each side of the diaphragm is tiny. Because these holes are small, if they're partially clogged with rust, dirt and slime, it can take some time for sufficient volume of water to flow through the hole to equalize the pressure and shut off the toilet fill valve.

If you replace the rubber diaphragm in your toilet fill valve, the problem will likely be fixed.