My toilet was leaking water continuously into the bowl. I thought I had a bad fill valve and replaced it with a new Fluidmaster fill valve. Unfortunately the problem persisted. When the float reaches the level where it should stop water flow it does stop water flowing out of the tube into the tank. However water till leaks into the tank in some manner which I cannot figure out. Eventually the water level will reach the height of the overflow tube and begin to spill into the toilet bowl. I have tried moving the float lower down to see what happens and found that water will continue to flow into the tank even to the point where the float is totally submerged. Any suggestions?
You need to turn off the water supply at the stop valve, gently prise open the Fluidmaster fill valve by gripping the valve "riser" (to keep it from turning) and rotate the upper valve housing CCW 1/4 turn, to open and tip it away from the riser, exposing the washer/diaphragm. Barely crack the supply valve open a bit to allow water to flow up the riser to flush debris out, using the flowing water to bathe the washer/diaphragm and flush away debris caused by your maintenance activities.
Reassemble and live in peace.