The flapper, which is a flat rubber stopper that covers the tank drain, may have become worn or distorted, allowing water to flow nonstop into the toilet bowl. If that flapper is intact, the flow of water could be coming from the fill valve, which controls the flow of water into the tank.
If the fill valve is broken or otherwise compromised, the valve will not close, allowing water to flow into the tank unimpeded. (Your tank will not overflow because it is equipped with an overflow tube, located in the center of the tank, that allows water to flow out of the tank when the level reaches the top of the tube.)
A continuously running toilet is a major cause of sky-high water bills, so if you decide to keep your toilet, you must get this problem solved as soon as possible.
Minor leaks can also be an issue with an older toilet. There are rubber gaskets inside the tank where the supply hose connects to the fill valve and the tank connects to the bowl. All of these gaskets can begin to fail over time, allowing water to escape from the bottom of the tank.
If water is leaking from the bottom of the toilet, there is danger of structural damage to the floor. There is a wax ring between the toilet and the floor drain to prevent leaks, and if you notice water on the floor or stains on the ceiling in the room below, you must get the wax ring replaced.
Toilet Replacement
If you intend to get the wax ring replaced, you may as well replace the toilet, because the toilet will need to be completely removed to replace the ring. You may be tempted to replace it yourself because the procedure seems relatively simple.
However, it's a wet, dirty and heavy job. You first need to drain the tank and bowl completely and disconnect the supply line and the two nuts that secure the toilet. You then need to lift the toilet from its perch on the floor drain and remove all of the old slimy wax from the bottom of the toilet and the floor drain.
After replacing the wax ring and wiggling the heavy toilet onto the wax ring, you must be certain the toilet is completely level before securing it to the floor.
As with many other deceptively simple tasks, theory and practice can diverge widely. It might be a better idea to leave installation to a professional plumber while you limit your participation to deciding whether to upgrade to a more opulent throne.
Toilet Upgrades
There are many practical upgrades you can choose from to make your private time as comfortable as possible. If you've noticed that your old round toilet seemed to shrink in size over the years (or maybe you grew a little larger), you can upgrade to an elongated oval toilet. You can also choose a high-efficiency model, which uses less water than older toilets.
You could even add a very cosmopolitan bidet function, popular in Europe, which uses a water spray for cleaning instead of toilet tissue. Express Plumbing & Drain will install a throne fit for a king or queen.
If you're in the San Fernando Valley area, Express Plumbing & Drain will install a throne fit for a king or queen.