American Standard has been manufacturing toilets since the late 1800s, and during that time its flush mechanisms have evolved. The one you're most likely to find on your toilet is the conventional flapper mechanism common to most gravity-fed models, but if your toilet is older, you may have an actuator. It has two plastic cylinders; one of these is sealed and one fills with water when the tank fills. When the toilet doesn't flush properly, the repair procedure is basically the same for both types, but if you need replacement parts, you must match them to your particular valve.
Flapper Doesn't Stay Up or Drops Too Quickly
Open the tank and check the tension of the flapper chain by lifting the middle with your finger. It should rise only about 1/2 inch before the flapper begins to rise. Adjust the chain, if it's too loose or too tight, by unhooking it from the flush handle, moving it one or two links in the proper direction, and reattaching it.
Examine the flapper, if your toilet has a conventional flapper-style valve, by unhooking it from the overflow tube. You'll have to shut off the water and empty the tank before you do this. Replace the flapper if it has any holes or tears, or if it's full of water. Take the flapper to the hardware store to find the proper replacement. In many cases, a universal flapper will work.
Clean the hole in the back of the open cylinder, if your toilet has a actuator. This hole is designed to let water flow out, lightening the actuator so that the flapper can fall, and when it gets clogged with mineral deposits, the flapper stays up. Use a pipe cleaner or piece of 14-gauge wire to clean out the deposits. You can do this without emptying the tank.
Check the chain tension and loosen the chain if it's too tight. It may be preventing the flapper from seating properly.
Replace the flapper. If your toilet has a brass actuator, unscrew the old flapper and screw on the new one. If the actuator is plastic, pull the old flapper off of the knob on the actuator and push the new flapper on.
Empty the tank and clean the valve opening with a scrubbing pad soaked with white vinegar. The vinegar will loosen mineral deposits on the rim of the opening that may be preventing the flapper from making a seal.