If the valve's brass take the cap back off and clean the brass stem on the bottom of the cap. I use a small screw driver and scrape all the calcium build up off of there so the plastic float can do it's job.
Does the float seat? Some of the floats are a different size and don't close all the way. Maybe use the old float with a new washer.
-
If the valve's plastic depending on which model valve you have take the cap back off and make sure the stem of the plastic float is correctly inserted into the hole of the cap. If you miss that hole it'll leak like crazy.
-
Is the O-ring or fiber washer for the cap missing or cracked?
I will check the movement of the float, I replaced the fiber washer.