Yes as posted by Fluidmaster support this will work 300 Fluidmaster model. I purchased 2 from my local home depot today. I easily removed the old washer and installed the 242. The squeaks, noises constant running of tank issue are all gone. See my pictures the wet one is the old damaged one and new one is on left. They are identical in size and appearance. Per Fluidmaster support: The 242 seal is used in all of our fill valves including the 300 model, that means that you can use it to try to fix your issue. If after replacing the seal you still have the same problem then the recommendation would be to remove (completely) the 300 valve from the tank and get the model 400A which is the direct replacement.
As the seal deteriorates it changes color and texture. This prevents it from sealing properly and the water leaks slowly at first before it becomes more constant. You can test this by turning the water valve off once the tank is full. Hours later your tank will either still be full or it will be empty.
It's usually the seal that gets worn out over time.
Yep. explained at the following URL https://helpfulcolin.com/2014/05/toilet-silent-fill-valve-works/#Basics
Probably. If you push the float down an inch or so (after the tank has filled) and let it pop back up, and the leakage stops, then the old seal is probably not shutting off the flow of water when the float slowly comes up. In this case ( which is what my valve was doing) a new seal should fix the problem.
I can't say for sure, but for lessthan $2 it is worth a try.
I know it worked for me. It was worth a $2 investment when I needed to fix my toilet. It was try this seal or replace the entire unit ($25-30), the seal was my answer.
after you remve the cap, you must also remove the plastic retainer. It has two tabs that you must pry up slightly, then turn the retainer CCW. The seal is under the retainer.
Yes, this fits the fluidmaster 400A, the mansfield isn't a flush valve it's the tower or flapper like thing. I had a friend who had a $700 Kohler toilet that kept running, he didn't know the fill valve was a fluidmaster(it looked completely different then the 400a model) but when I took it apart it was the same washer inside. I have a feeling this is used in the entire line of products.