Normally, your toilet bowl fills with water only after you flush the toilet. When you press on the toilet's handle, an arm inside the tank lifts up. The arm pulls on a chain, which lifts the flapper valve at the bottom of the tank, allowing the water in the tank to drain into the bowl. A bad flapper valve will cause water to flow into the toilet bowl constantly, as will a bad fill valve in the toilet's tank.
Lift the toilet tank's lid off the tank and place it somewhere safe. Inspect the chain that attaches to the arm at the top of the toilet tank as well as the flapper valve. Straighten the chain if it has bound up anywhere.
Adjust the flapper valve's chain if the chain does not have about three links of slack when the flapper valve is closed. Slide the chain off the clip on the end of the arm, and then slide a different link on the chain to lengthen or shorten the chain.
Press on the flapper valve inside the toilet tank with a ruler if the bowl continues to refill constantly. If the toilet stops refilling the bowl, the flapper valve must be replaced.
Turn the handle on the toilet's water supply valve clockwise to shut off the flow of water to the toilet. Press the toilet's handle to flush it once, and then vacuum out the rest of the water from the toilet tank with a shop vacuum.
Twist the old flapper valve off the posts on the base of the fill valve and slide the chain off the clip on the arm at the top of the tank. Press the new flapper valve onto the posts on the base of the fill valve and slide the chain onto the clip on the arm so the chain has about three links of slack. Turn the fill valve's handle counterclockwise to refill the toilet tank.
Adjust the water level in the toilet tank so it sits about one inch below the top of the overflow tube. Too high of a water level causes the water to flow down the overflow tube, while too low of a water level does not allow the fill valve to turn off.
Bend the arm holding the float down. For floats that do not sit on the end of a metal arm, pinch the metal clip on the float or turn the screw counterclockwise, allowing you to slide the float down.
Pull up on the float inside the toilet's tank while the toilet is refilling if the flapper valve is working correctly. If the water continues to flow, the fill valve is the source of the leak. Drain the water from the tank by closing the water supply valve and flushing the toilet.
Turn the nut on the end of the toilet's water supply line counterclockwise with adjustable pliers, and then turn the lock nut on the bottom of the fill valve counterclockwise with the pliers until it comes off. Lift the fill valve out of the toilet's tank.
Twist the new fill valve's top to adjust its height so it stands one inch taller than the overflow tube in the toilet's tank. Insert the new fill valve into the hole in the tank and then tighten the lock nut onto the bottom of the fill valve before you tighten the nut on the end of the water supply line attached to the bottom of the fill valve.