I wonder if this is the wrong way to address a toilet that leaks and refills every so often. I have an American Std 4117A toilet that refills for 5 or so seconds. I think it is leaking around the flapper. It has a fluid-master 400 anti siphon float valve but a 540 flapper with a plastic cup under the flat part that actually sits in the drain hole. I have a broken hip so going out hurts for a replacement. So, I detached the cup (carefully as there are plastic extrusions to keep it where the engineers wanted it, I guess) and dropped a 3 oz fishing sinker in it and reattached it. I tried this in a different way once to rule out the flapper, at least, but it worked in that past instance. I'm just wondering if I am tempting the toilet gods. Hank
The 540 flapper has had a problem with the white silicone disk on the underside that actually seals the hole. Some develop bubbles on the surface that prevent a good seal. I've had some simply fatigue and not maintain the gap between the seal and the body of the flapper that allows it to conform to any irregularities.
They have a 10 year warranty. Even if you can't prove date of purchase, this is a known manufacturing defect issue, and Fluidmaster has been sending a set of free replacement seals if that is the issue. They have an online live chat for product support that is an easy way to contact them: https://www.fluidmaster.com/support/
The seal fits in a groove at the base of the dome where you added the weight. It's stretchy, and you just pull off the old one, then work the new one to the groove and make sure it cleanly embeds in the groove all the way around.