How To Replace the Fill Valve
Inside the tank is a water valve that controls the water inlet for the refill cycle. When the toilet is flushed, the water is released from the tank and the float sitting on top of the water, drops down, engaging the fill valve to release water into the tank. Depending upon who you ask, you might hear it called a "fill valve", "flush valve", "water valve" or "tank valve". It most certainly is not the flush valve, because that is the part involved with releasing the water at the bottom of the tank. The most correct name is the fill valve, although it is a water valve and it is located in the tank, but from here on, we'll refer to it as the "fill valve".
If the fill valve is misadjusted or defective, a common symptom is the periodic or continuous running of water. It may sound like a flush, a mini-flush, running water or brief refills. These symptoms can also result from other problems, primarily leaks.
To replace the fill valve, remove the lid from the tank and set it carefully onto a towel. Vitreous china can easily scratch other surfaces, so the towel is highly recommended. Turn off the water supply at the valve behind the toilet, or at the main valve if the toilet doesn't have its own shut-off. Flush the toilet to drain the water out of the tank. Place a towel on the floor under the tank to catch any spillage.
Inside the tank you will see a tall column with a float device connected to it. The valve is fastened through a hole at the bottom of the tank. Under the tank there is a large nut that secures the valve to the tank. Loosen and remove that nut and the valve should lift out. Disconnect the hose that runs from the valve to the overflow tube.
Clean the bottom of the tank around the valve hole. Place the washer onto the new valve and set it into the hole. If the new valve is height adjustable, adjust it now for your tank. Place the second washer onto the valve under the tank and tighten the nut to secure the valve in place. Do not over tighten the nut or you may crack the tank. If the manufacturer's instructions differ from these, always follow the manufacturer's instructions.
If you are replacing only the fill valve with a new part, you may need to reuse the float mechanism from the old valve and fasten it to the new part. Adjust the float so that the water level will be lower than the top of the overflow tube.
Turn on the water and check to see that the valve is refilling the tank. Watch until the tank is filled and the valve shuts off. Inspect under the tank for leaks. Now flush the toilet and watch the complete cycle for proper operation. Replace the tank lid and you are done.