2 possible solutions: take it apart clean, inspect, replace any seals,
or buy a new fluid-master install adjust.
Both are messy, as after you turn the water off and flush it, and
sponge up any remaining water, there still always seems to be some drip
from the mounting hole after the old one is removed. just have a catch
can and rags handy. This is an ideal time to inspect the feed line and
valve. if you have no valve and had to turn off the main water valve to
service the toilet, now is the time to add one. If the end on the
supply valve isn't compatible with the new valve (assuming you op for
the new valve) it would be a good idea to pick one up with the new
valve, too long is OK if you don't mind the plumbers loop to take up
the excess, sometimes they don't have the exact length as your old one.