To replace the seal in a Fluidmaster fill valve:
- Shut off the water to the toilet. To get to the seal you will have to take apart the fill valve and for that the water needs to be off.
- Hold the fill valve shaft. Push the float up towards the top of the fill valve. While holding the shaft body with one hand use the other hand to turn the cap on top of the fill valve counter clockwise 1/8 of a turn. Pushing down on the cap while turning can help it turn easier. The top should pop off and you can then get to the seal.
- Replacing the seal. Just lift out the old seal and make sure that the inside of the Fluidmaster fill valve is clean before putting the new seal in place. Then just insert the little hole in the seal onto the needle like stem and push it flat all the way around.
- Put the fill valve back together. A ⅛ clockwise turn while holding the body of the fill valve will lock the unit back together. Then you can turn the water back on and check that the new seal fixed the problem.