A new "Float Master" toilet filler was installed. After flushing, the float will not drop down on it's own so the tank can fill back upi with water.

You may have the water level set so high its forcing the float so high it gets stuck. If you still have the box with directions it will tell you how to set the height. The first step when installing a new device is to cut the plastic over flow tube to the correct height (usually by matching the old ones height). This is what will regulate how high the float can go before the water over flows into the pipe and then there is a fine adjustment on the float itself that's like a twisting screw like thing but it has limited movement.

Usually the overflow tube does not get replaced when the fill valve is installed. Did you install following direction or these steps How to Replace the Float Valve in a Toilet Tank: Bathroom Maintenance | eHow.com (http://www.ehow.com/video_5116755_replace-float-valve-toilet-tank.html).. Make sure the float is not touching the side of the tank, if it toouches rotate the entire valve.

Assuming that you mean Fluid Master, not Float master since a Google of "Float Mater" only gave them Fishing lures and camping supplies.

Make sure that the cap and the lever to which the float is attached is properly assembled. Maybe someone opened it before you purchased it and did not put it back together properly. The lever must be in a notch.