The flapper in the tank is bad, the water level is too high and it is running down the overflow tube, the fill valve need a new washer in it. One of these the problem, you will just have to find out which. Most likely is the flapper.
Make sure you have water going through the line.Also check the float in the tank to be sure it is not at the top.Hope this helps.
Watchdog
Ans 2 - Fill valves don't last forever, and if it's over 10 years old it's best to replace it anyway. Buy a simple Fluidmatic A400 or Fluidmatic Pro, (both around $16-18), anda new universal flapper (they wear out too) Turn off water at the incoming floor valve and remove the pipe to tank, then fill valve itself. Takes less than an hour to replace both parts and is well worth it. While the pipe is off the floor valve, test it by opening it a bit with an old towel around it to catch water. These DO get clogged and I replace many of them (also easy to do)
Float level is too high or minerals have built up in the fill valve and are keeping it from shutting off. If the fill valve is in the corner of the tank and the float rides up and down on it, the top come off and you can clean the build up out. Turn the water off, hold the shaft of the valve and turn the top, 1/8th turn counter clockwise and the top should lift off. You should see a quarter size black washer and it will probably have what looks like sand on it. This should wash off and solve the problem.
The flapper in the bottom of the tank needs to be replaced or the chain going to it is too short.
change the flush valve
Assuming you have a regular tank toilet with a ball float and rubber flap and that the water is turned on and the tank is filling, the most common cause of a toilet not flushing is a broken or detached chain that runs from the handle lever to te rubber flap that keeps the water in the tank.
Toilet tank
If you mean the toilet tank, it means you have a leak and water (and $$$ if you have a water meter) are being lost. It is the same as a leaky faucet. You need to find the source of the leak and repair.
A vacuum breaker is a device that prevents water in a toilet cistern or water tank, from syphoning back into the toilet cistern or water tank.
Each flush of a toilet uses the same amount of water.
Flapper in tank sticking or not seating properly. Needs replaced.
seal to stop water from runing into tank located between tank and bowk
Yes, your problem may very well be beyond the tank the water is not escaping fast enough out of the tank, to cause the water to rise at the less restrictive point, look beyond the tank make sure the water is coming or going out of the tank unless you have never had the tank pumped out, but in most cases the line beyond the tank is crushed or blocked.
Install a tempering valve in water line feeding toilet. Tempering valve will mix hot and cold water together so warm water fills toilet tank which will stop condensation.
It's the Rubber seal at the bottom of your water tank, when you flush the rubber seal raises and the water flashes your toilet. Generally its black but not always.
NO.
If your water comes from a well and you have your own pump and tank. The tank may be waterlogged.