A fluid master type fill valve has an integral float that has an adjustment rod on the side of the assembly. You should be able to pinch the spring clip and adjust the float up or down. Move it down a half of an inch for a toilet that is running constantly. Let it go and see if the water stops running.
This tutorial describes how to adjust different types of toilet fill valves,.
As the water level in the tank rises and falls, the float rod and ball move . The ball in the toilet is a water fill regulating device. Depending on the age and style of toilet that you have, floats come in two basic styles: a stand-alone ball float or a cylindrical float that is an integral part of the . The water level in the tank is controlled by an adjustable float. To adjust your toilet tank water level: If your toilet has a float arm, use the adjustment screw on top of the fill valve to raise or lower the float arm.
Older float arms may not have an adjustment screw and should probably be replaced with a newer fill valve. However, you can adjust your toilet float to solve some of these common issues. Adjusting will only take minutes of your time.