The water from my Fluidmasrer 400
runs fine. The problem is it doesn't
float up and down. What could be the
problem?

Calcium build up can prevent the float from moving up and down, if it's not correctly positioned it can hang on the side or back of the tank, or it may just be worn out.

The Fluidmaster 400 fill valves are less than $10, just replace it.

Correction. The float goes down but not up.
Would I need to replace the whole
unit or just the uppper
part not including the shaft which
screws into the tank?
Thanks

Turn the angle stop valve off under the toilet and flush it holding the flapper up until it won't drain anymore. Now take a sponge and remove the rest of the water, about 1/2" deep on the bottom until dry. Then disconnect the Nut on the bottom of the toilet water connection. Now place a towel down under the angle stop and remove the nut that holds the ball cock in place and install a new Fluid Master ball cock. Read the installation instructions so that you expand the Fluid Master to the right height so is will fill the tank to the proper height. Now you should be good to go because you did what a Master Plumber would do. Good luck

EDIT: Packy's remedy below would work also but be careful when removing parts inside a tank. Some can break easily...just be gentle but firm and don't put pressure on the handle or the overflow for best results.



Edited 2 times.

you can change just the top.
look at the new valve. there is a clear (or grey ) sliding plastic piece that looks the top into the shaft. slide it down and the top 1/2 will pull up and out. remem ber to lock the new top vbefore turning the water back on..