The Fluidmaster Fill valve (Ballcock) can suddenly quit filling or start filling very slowly. It happens to be very sensitive to crud in the water supply and it is easily repaired.
To clean and return fill speed back to normal follow these easy steps.
1. First, turn off the water supply to the toilet and check by flushing the toilet and insuring no water is coming to the valve.
2. Grab the valve stem just below the float with one hand and
3. Lift up on the float arm on the top of the valve and twist 1/4 of a turn counter clockwise.
4. The head can be tilted enough to remover the gasket and check for tears.
5. Now with your hand or a cup cover the top of the exposed valve stem and turn the water back on.
6. Turn the water on just enough to purge some water out of the valve, this will purge any rust or impurities that are interfering with proper operation.
7. Reassemble the valve head, with the old washer or new, and turn the water back on.
Your toilet should fill like normal again!