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How to repair toilet inlet valve? (Fluidmaster 400UK)

Hi, I currently have two toilets with what I believe are perished seals on the inlet valves. Both toilets have the same type of inlet valve, Fluidmaster 400UK (photos below).

Firstly, I just wanted to confirm that I have correctly diagnosed the issue. The inlet valve in the first toilet is leaking constantly from near the top of the valve and causing the water to constantly trickle down into the bowl via the overflow. The inlet valve in the second toilet makes a high-pitched whistling sound as it refills the cistern.

Secondly, I want to ask what the best solution is. I was planning on just replacing the valve seals with the Fluidmaster Toilet Inlet Valve Replacement Seal (https://www.bunnings.com.au/fluidmaster-toilet-inlet-valve-replacement-seal_p4926915). However, upon reading the reviews it seems as though many people have had issues with these replacement seals not providing a proper seal. Can I use the above seal on the model of inlet valves I have or am I best off replacing the whole unit? And if I do replace the whole unit, is the Fluidmaster Bottom Entry Toilet Inlet Valve (https://www.bunnings.com.au/fluidmaster-bottom-entry-toilet-inlet-valve_p4926949) the appropriate model to use?

Thanks.

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JacobZ

Hi @mitch0816,

It certainly sounds like you’ve diagnosed the problem correctly. Both toilets are showing classic signs of inlet valve seal failure.

In the first toilet, where water is leaking constantly from near the top of the valve and trickling into the bowl through the overflow, the issue is almost certainly a worn or perished diaphragm seal inside the Fluidmaster 400UK valve. When that internal rubber seal deteriorates, it can no longer shut off the water flow properly, so water continues to pass through the valve and overflow into the bowl.

In the second toilet, the high-pitched whistling sound during refill is another common symptom of a seal that’s starting to harden or break down. As the seal loses flexibility, it doesn’t regulate water flow smoothly, and air or turbulence inside the valve creates that whistling noise. In some cases, a small piece of grit or scale stuck under the seal can cause the same sound, but it almost always points to a worn internal component.

You can try replacing just the Inlet Valve Seals; however, while the seals themselves are compatible, success varies because the plastic housing and internal surfaces of older valves can become worn, pitted, or coated with mineral buildup. Even with a new seal, the valve may not seat properly and could still leak.

If your valves are more than around five years old, it’s usually best to replace the entire inlet valve rather than just the seal. The Fluidmaster Bottom Entry Toilet Inlet Valve is the correct replacement for the 400UK and installs easily in the same position.

To fit it, simply turn off the water supply, flush the cistern to empty most of the water, disconnect the hose, remove the old valve nut from underneath the cistern, and fit the new valve in its place. Tighten the nut securely, reconnect the hose, turn the water back on, and adjust the float to set your preferred water level.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob

mitch0816

Hi Jacob,

Thanks for your response and advice. I will replace the entire valve as suggested.

Also, we are finding that one of the toilets has a weak flush. Do you think this could be a result of a faulty flush valve?

If so, do Bunnings sell replacement flush valves also? Here is a photo of the valve for our toilets.

Cheers,

Mitch

MitchellMc

Hi @mitch0816,

Replacing the outlet valve could resolve the weak flush issue. Could you tell us a bit more about it, though? Is it just not releasing enough water, or is it slow to release? Does it dispense the whole volume of water contained in the cistern?

From the image, there doesn't appear to be a large volume of water in the cistern, though the water level seems to be just below the overflow pipe. There might be some adjustments on the valve to allow more water into the cistern. However, the toilet might be designed to be very water-efficient. Water efficiency sometimes leads to weaker flushes.

The first thing to check is whether, when the toilet is flushed, the entire volume of water contained in the cistern is released. Try pushing the button once and letting go. Then try pushing the button and holding it down to see if that releases more water.

Please let me know if you have any questions.

Mitchell

mitch0816

Hi Mitchell, great name.

Thanks for the response. I am currently in the office but can do a bit more investigating tonight. However, from memory I think the water level is about an inch or so below the overflow pipe.

I also think that not all the water is being released from the cistern upon flushing. Again, I can't confirm that at the moment but fairly sure that's the case. Assuming that is the case, would that likely mean that the outlet valve requires replacement?

If so, are they fairly generic for all toilets or do I need to make sure I get a specific type?

Thanks again,

Mitch

MitchellMc

Hi @mitch0816,

Many modern toilets have an adjustable setting on the outlet valve that controls how much water is released. Sometimes designed as a water-saving feature or to allow for half flush/full flush operation. It’s worth checking whether your current valve has a small adjustment tab or slider that can be moved to allow more water to flow into the pan.

Outlet valves are generally fairly universal, but it’s important to choose one that matches your toilet’s button mechanism. It looks like you have a single push-button system, in which case you can typically replace it with an off-the-shelf universal outlet valve from brands like Fluidmaster or Kinetic. The Kinetic 17 - 25mm Single Flush Outlet Valve could be an option. If you can identify the brand or model of your toilet, that’s ideal and will ensure a direct fit, but otherwise, you can usually match it up visually when purchasing.

So yes, if the water level is correct but the flush isn’t clearing properly, start by checking the outlet valve’s adjustment. If that doesn’t resolve it, a replacement outlet valve (matched to your button type) should do the trick.

Mitchell

mitch0816

Great, thanks. I will take a look tonight.

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