Fluidmaster fill valves are a common type found in many toilets. The Fluidmaster fill valve uses a plunger that rides up the valve shaft rather than an old-style ball and rod. Debris and sand in your water system can cause the seal inside the fill valve to not close properly. When this happens, the fill valve continues to allow water to run. Repairing a Fluidmaster that does not shut off requires you to flush the valve.
Turn the handle on the water shut-off valve behind the toilet clockwise to stop the water flow to the Fluidmaster valve. Pull the toilet tank lid off the toilet and set it down on a flat surface away from your work area.
Hold down the flush handle while you flush the toilet. Allow as much water as possible to drain from the tank before you release the handle.
Grab the bottom of the Fluidmaster fill valve with one hand and grasp the top with your free hand. Lift the plunger arm and turn the top of the fill valve counterclockwise while lifting up on the top. This removes the top from the valve. Allow the top to hang over the side of the valve.
Hold a cup 3 inches above the top of the valve. Turn the water supply on and off several times. This flushes out the system. Turn the water supply off and look at the inside of the top cap. The seal in the cap shouldn't have any cracks.
Replace the seal if there are cracks or other damage. New seals are available at hardware stores. Push the top cap over the top of the valve and turn it clockwise so the plunger arm is straight. Turn on the water supply and wait for the tank to fill up to the water level line on the inside back of the tank.
Adjust the float valve if water is still flowing from the fill valve after it reaches the water level mark on the back of the toilet tank. Pinch the clasp together on the plunger arm and raise the plunger. Set the toilet tank lid on top of the tank.
If water still continues to run, replace the Fluidmaster fill valve.