Shut off the water to the toilet by turning the shut-off valve all the way to the right. The shut-off valve normally is on the end of a supply pipe that comes out the wall behind the left side of the toilet. Remove the tank lid. Flush the toilet and hold down the handle to drain as much water as possible from the tank.
Lift the arm of the Fluidmaster 400A. The arm is just below the cap of the fill valve, and is attached to the vertical stainless steel link of the fill valve. Do this if the Fluidmaster 400A runs non-stop without shutting off. Press down on the valve cap and turn the cap and the arm counterclockwise 1/8 turn. Lift the cap off the valve.
Hold a small cup directly above the uncapped Fluidmaster, while twisting the shut-off valve to the left for a few seconds and back all the way to the right, several times. This sends water up through the Fluidmaster, flushing out debris and rust. Turn the shut-off valve completely to the right. Place the cap on the fill valve and twist the cap 1/8 turn clockwise. Turn the shut-off valve to the left.
Pull the refill tube, which runs from the nipple of the Fluidmaster 400A, out of the top of the tank's overflow pipe. Do this if the fill valve constantly turns on and then off again. Clip the angle adapter on the end of the refill tube onto the top rim of the overflow pipe. If the water continues to run non-stop, proceed to the next step.
Squeeze the water level adjustment clip that is located on the vertical link of the fill valve. Slide the clip and the valve's float cup down the link, about an inch at a time. This lowers the amount of water that will fill the tank.
Flush the toilet and observe the tank's water level. Squeeze the clip and lower the cup, as necessary, to bring the water level even with the mark designating the proper water level on the tank wall. If there is no mark, a good water level is between 1/2 inch to 1 inch below the top of the overflow pipe. Place the lid on the tank.