I have a fluidmaster model 350 and I notice that my tank fills fine and everything but the float cup, at the very end near when the tank is near full it closes semi-abruptly which causes a bang in the wall.

It isn't a constant water hammer type of thing, but a kind of loud thud in the wall that happens once per flush. I observed the float rising with the tank lid off and at the very end just before being filled up, the float cup rises quickly/abruptly and that causes a thud in my wall. I tested by manualy holding it down to let it raise more gradually by hand and if I control the rate of speed by which the float cup finally rises to it's highest (fill) point, the thud in the wall DOES NOT happen. If I let it rise on it's own, the thud does happen. I observed the fill cup rising gradually then at the end it makes that sudden float upward and that causes the bang.I tried moving the water level adjustment rod up and down to see if it would help and a couple of times when screwing clockwise, it did help but then the thud came back on it's on on subsequent flushes. I observed my other toilets (which are used less frequently) and the float cup rises evenly throughout the whole tank fill process and thus no thud from those toilets.

Should I just replace the whole fill valve rather than fiddling with the leveler screw and/or using a water arrestor?

I appreciate any advice on this. Thanks!

  • I "fixed" this in an apt by simply cranking down the exterior fill valve under the tank so that it filled slower and snapped off slower; didn't fix it 100%, but was probably 75% quieter...
    – dandavis
    Jun 15, 2023 at 18:28

2 Answers 2


You didn't say how old the current flush valve was. Mine was about 5 yrs. I had this same issue. I tried to clean the rubber valve and reduce the water flow to make the tank fill slower, but that reduced the water in the bowl and after 3 flushes the bang was back.

In the end I replaced the flush valve with new and the issue is gone. Has been for almost a year.

  • Yep, sorry about that. The whole toilet and guts are 3.5 years old. Will be 4 in January of 24. So you think I should go with the arrestor or new fill valve?
    – user168330
    Jun 15, 2023 at 16:26
  • 1
    Arrestor or fil valve? That is opinion and not allowed here. However if the toilet didn't not bang before, one would assume that an arrester would not be needed, but the cause of the bang should be corrected. Changing the fill valve is substantially easier than installing an arrestor. Therefore my decision would obviously be change the offending part.
    – RMDman
    Jun 15, 2023 at 16:31
  • could this also be water pressure related? My toilet has a push start valve to control water flow and I pulled it to turn the water off and flushed, then pushed to let water back in and when the tank was full it did not thump in the wall. Issue is, how to control water pressure to a single toilet if its the push/pull valve?
    – user168330
    Jun 15, 2023 at 20:59
  • I'm not familiar with a push start valve, please explain. My answer was based on personal experience with my own toilet and others I changed flush valves in for the same reason.
    – RMDman
    Jun 15, 2023 at 22:56
  • when turning the water off to the toilet, mine doesn't have the twist and turn to where I can close the water part way. I have a little pull/push water stopper that you push to open water and pull to stop water. So there's no inbetween pressure, it's all or none. And like i mentioned, I turned the water off to the toilet, flushed, then turned back on and the thump in the wall did NOT happen. But it comes back after a while. I guess replacing the fill valve is cheaper than the arrestor so maybe I will do the fill valve and if it persists I will add the arrestor. Thanks!
    – user168330
    Jun 16, 2023 at 13:33

A water hammer happens when water flow stops, eg once per flush exactly as you have it. I wouldn't dismiss that so quickly. A thud in the wall is a possible symptom.

Here are some easy suggestions from Fluidmaster. To me they seem a little desperate and unlikely to work, but they are easy and free to try.

Install a hammer arrestor behind the toilet near the valve. IMO it's low cost, high chance of success, the best first thing to try.

Looking for a fill valve that stops more gradually will be a frustrating trial-and-error exercise, if such a thing even exists. The old (1970s) lever arm piston valves closed more gently, but I don't think you can buy those any more. Modern diaphragm valves all work similar ways. Maybe one brand closes more gently, but they aren't designed or marketed that way so you would just have to keep buying them hoping one will do what you want, and I think you'll be dissapointed.

[Edit .. responding to comment]

Your other toilets don't do this? If their valves are a different model to this one it would be a reasonable first step to buy one of those. Or to swap them, and see, does the problem move with the valve? Or is the problem fixed to this toilet in which case it might be your problem is just a loose pipe in the wall behind this toilet, and a hammer arrestor will hopefully help. Worst case you need to break the wall and fix the pipe to a stud. Swapping the valves of two toilets could be easy (if they are not too old, the washers are all in good shape, and the stopcocks all work) or a difficult job (if the stopcocks are old and don't function, the washers are disintegrating, etc).

  • thanks for the advice, I was on the fence about which path to take. Ill try the arrestor. Do you know why my other toilets floats/fill valves close more gracefully and don't cause the bang int he wall? Also, in the off chance that the arrestor doesn't work, what would be the next step to take? Thanks!
    – user168330
    Jun 15, 2023 at 16:10
  • Replied to comment in answer.
    – jay613
    Jun 15, 2023 at 16:22
  • Hi @jay613 all fill valves are the same age (part of a new build in January of 2020) and all are the same fluidmaster 350 models.
    – user168330
    Jun 15, 2023 at 16:27
  • Also, is this a good arrestor: Sioux Chief Mfg Sioux Chief 660-TK Female Swivel Nut by Male Ballcock Thread Mini Rester, Brass/Antique Brass ?
    – user168330
    Jun 15, 2023 at 16:28
  • I added a link above to a support page from Fluidmaster. Worth a try. Sorry but I can't help you choose an arrestor except to say there are some with 3/8" compression connections so they fit between the stop valve and supply hose. Couldn't be easier.
    – jay613
    Jun 15, 2023 at 16:40

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