Fixing Leaking Toto Washlet S300 / S350 Junction Valve

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Is your Toto Washlet S300 / s350 Junction Valve leaking at the tank connection?

I had this problem and tried many different combinations of fiber washers, o-rings, conical rubber gaskets, flat brass washers, sealant tape, metric thread adapters, etc. All to no avail. The final configuration that worked was not intuitive, but is absurdly simple.

TLDR; Go get a variety pack of washers at the hardware store and add an extra thin FLAT brass washer to the mix.

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Step 1: The Official Toto Instructions Say to Use Only Flat Fiber Washers to Seal

The official Toto instructions DID NOT WORK in my case.

If the connection to your tank float intake end is PERFECTLY FLAT, then you might be able to follow the official Toto instructions and have the fiber washer alone seal correctly.

I even purchased a new toilet tank float intake and another valve set thinking my original THU9339 Junction Valve was defective, that the THU9340 Fiber Washers were bad, and/or that I had stripped the threads on the old float assembly.

Step 2: Inspect the Fill Tank Connection End

If the fill connection to the tank float intake is CONICAL on the inside, then you may need to install an extra gasket to avoid leaks.

Skip ahead for the solution, but for those who are interested, here is what I tried:

  • I reseated the connector, careful to not cross-thread or strip the plastic tank connector. Still leaked
  • I added thread-sealing tape and tightened the female nut down with a crescent wrench as tight as I could. Still leaked.
  • I tried using a flat rubber washer on top of the fiber washer. I barely could get the threads to grip and it still leaked.
  • I used a conical rubber washer that fit perfectly inside the bevel of the tank connector but could not even get the thing to grab the threads (too far away).
  • I removed the fiber washer and tried the conical rubber straight against the flat brass insides of the T connector. Still leaked.
  • I bought a new tank float as the threads on the old one were a bit mangled by now. Still leaked!

  • I bought a NEW Toto T connector and repeated the above steps. STILL LEAKED!

By now I had purchased every kind of adapter and type of washer I could find and had brand new parts to test without having to be upside down wedged between the wall and toilet.

I'm still blown away that Toto would provide such a user-unfriendly valve and completely inadequate instructions. But I digress... :)

Step 3: Use an Additional FLAT Brass Washer on Top of the Fiber Washer

*** Add an additional FLAT brass washer on top of the fiber washer. ***

Instead of following the official Toto instructions:

Junction Valve > Fiber Washer > Tank Inlet...

It should be:

Junction Valve > Fiber Washer > Brass Washer > Tank Inlet

This creates a flat surface for the toilet tank inlet to sit on without adding so much space that the nut cannot be screwed on far enough to seat correctly.

Now attach the Junction Valve to the toilet tank as normal.





The fiber washers work by expanding slightly when they contact water, creating a seal that won't crack and leak like rubber washers do over time.

Just hand tightening made a good seal, but I added a 1/4 turn with a wrench to make sure. I did not use any teflon tape in any parts of the assembly. I've read that teflon tape is only to be used in non-compression fittings.




UPDATE 2018-08-05:

@chjwolters mentioned not using the fiber washers at all, seating the tank connector directly against the flat brass insert inside the gray plastic 'T'.

I think the key to success here is having a good surface for the plastic connector to seal against. I noticed that my fiber washer had a ring dented in it from the beveled edge of the tank connector, which gave me the idea of the brass washer. The brass washer might even out the pressure, allowing the fiber washer to do its thing.

@LeetPete successfully used "a combination of a flat rubber washer followed by a cone washer" to stop the leak. Perhaps the rubber cone that came with my variety pack was simply too tall.

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    27 Discussions


    7 weeks ago

    Man, you saved my day.
    i spent the whole morning to solve the problem until i saw your post.
    i rush to the home dept and bought the extra brass washer , and follow exactly the way you suggested, problem solved!!!! i almost decided to return the TOTO wahlet!
    one more thing i notice is that, when you connect to the tank inlet, making sure it is all straight up, you can feel it reaches the very bottom until you can not turn.


    5 months ago

    Purchase the TOTO THU9090R 6-Inch WASHLET Hose Extension Kit for C110/E200/S300/S400 WASHLET. If you are using a non-Toto toilet w/ the seat the threading is off which is why you have leaking. I tried all above and the adapter took minutes vs. hours of solving this problem.

    Screen Shot 2019-05-21 at 2.56.08 PM.png
    Jeff IL

    8 months ago

    Thank you so much for posting this information. I recently purchased the Toto S300 and spent half my Sunday trying to resolve this leaky junction valve. Your recommendation for the flat brass washer worked great and saved me a lot of hassle.

    I gotta say that I am really disappointed in Toto for this poor valve, lousy instructions and poor customer service. When I called Toto to resolve this they said it had to be the intake on my toilet tank that needed replacing, that it was worn out and wouldn't seat properly. If I hadn't replaced that part less than six months ago that would've been the route I took. As it is, I googled the issue and came across this thread.

    Thanks again.

    1 reply
    SemiconductedJeff IL

    Reply 5 months ago

    I had the same experience with customer service - following their instructions to replace with a brand new intake didn't change anything. Glad this worked. I'm also surprised that so many people need to do different approaches to stop the leaks. Must be a bad design with their T connector.


    5 months ago

    None of what everyone posted fully worked for me. The closest I got to a proper seal was using a brass washer then a flat rubber cone washer, but I was still getting a drip every 5 minutes or so.

    I called up Toto and they sent me a replacement T connector, but not identical to the one that came with my C200 (which was mostly metal), but part #THU6234, which is mostly plastic. It also has some sort of integrated cone-washer-type connection on the top. Screwed that on, only needed to hand tighten, and I have not had a leak yet. I also replaced the fill valve with one that had a brass bottom (Fillmaster Pro45b) to make absolutely sure I wasn't stripping the connection. I don't know if that had a factor, but it was still leaking with the brass bottom and old connection. So I believe there was some sort of minor defect with the metal T connection that kept on letting water out.

    1 reply

    Reply 5 months ago

    Toto has finally engineered a fix - SUCH good news. Thanks for sharing.

    I'm sure Toto's customer service has been getting hammered over this issue and still surprised it has taken more than a year to solve.


    1 year ago

    I'm so thankful for this thread! I just purchased the Toto S550e and am having the exact same leak problem! So far I've tried the washer that comes with the washlet and not using a washer at all, but no luck. I'm going to go to Lowe's tomorrow to buy a brass flat washer. I really hope this fixes the problem. Such a shame that Toto doesn't include some sort of solution in the box, especially for such a high-end bidet.

    5 replies

    Reply 1 year ago

    Update: I fixed it! Adding a brass washer didn’t do it for me though. I had to use a combination of a flat rubber washer followed by a cone washer. This was the only combination that would make the connection between the tri-connector and the tank inlet tight enough. I hand tightened it and then used a wrench for a little more tightness at the end. Ultimately, this ordeal cost me several hours of time, some back pain, and $7 that I spent for a variety pack of washers from Lowe’s.


    Reply 1 year ago

    LeetPete, thanks a ton for your advice on this.

    I too purchased a S550e and encountered this leak issue during installation. I was close to calling a plumber but decided to use your idea to combine the flat rubber washer (the official Toto one included with the box) followed by a cone washer. It worked!

    For those wondering what cone washers I used, here's the home depot link:

    Now my Toto S550e is working like a charm. I hope Toto is able to address this issue since it seems like many customers encounter this leaky pipe.


    Reply 10 months ago

    This worked!!! Can't tell you how frustrating it's been with this thing. I thought I had a bad one, so replaced it to no avail and then tried replacing it a second time. What's with Toto on this? They really have to sort that thing out. So glad I found this thread. Went to Home Depot and got the cone washer and it worked perfectly.Thanks guys!!!


    Reply 10 months ago

    So happy to help! Hope you and your bum enjoy your new Toto!


    Reply 1 year ago

    Thanks for sharing your solution!

    That method still leaked with my particular setup, though the water pressure in our building is also industrial-strength.

    Was the rubber cone washer you used the thick, flat-bottomed kind or the 'floppy' thin red ones? The flat-bottomed rubber cone washer in my variety pack may have just been too tall to allow the threads to seat properly.

    Most of all, glad you were able to solve it. :)


    Question 10 months ago on Step 3

    I'm so glad to find your post. I was desperate to fix the leak at toilet float intake and junction valve for Toto B100 (Discontinued model). I'll go get the Brass washer, but can you let me know what size of the washer (dia. and thickness) you installed? Thank you.


    Question 10 months ago

    Thank you for this page and everyone's input too.

    I hope you can help with my situation.

    My Toto is leaking from the top of the plastic part and the bottom of the top metal part of the Toto supplied junction.

    At one point it was right at the top where the metal part attaches to the inlet part of the toilet but now it's a leak within the junction parts only.

    Any ideas to solve that leak?

    When I attach the washlet hose to the plastic part of the junction I can see a droplet of water squeeze out between the plastic part so I know already it won't work.

    The junction is attached straight to the tank inlet.


    1 answer

    Answer 10 months ago

    Ok me again!

    I think I got it!

    7/8" Brass washer on the bottom... Then Toto supplied ring washer... Then cone washer... Then into the tank inlet.

    It seems to be holding now but I'll keep the water off tonight and check again in the morning.

    Thank you for all of your ideas!


    10 months ago on Step 3

    Same issue... I resolved with a rubber cone washer, I think the exact washer or combination of washers you need probably depends on the shape and condition of the filler valve sealing surface. The fiber washer failed on mine because the sealing surface on the filler valve was cone shaped and the bottom edge of the cone was not perfectly flat. Also, be careful about cross-threading, the tangle of semi rigid hoses and the awkward position you probably need to get into to see what your doing make it easy to cross-thread the stainless nut on the nylon thread. This can lead to little nylon shavings interfering with the seal.


    11 months ago

    Thanks to everyone here for the help. The folks at Toto are useless, and I sent them to this thread for more info if other people call in.

    Solved with the cone and flat washer combo suggested below (and purchased at Home Depot for $4). Everything is working smoothly right now and I cannot thank y'all enough.


    Tip 1 year ago on Step 3

    I had the exact same problem but what worked for me was not using any washer at all. For some reason, this was the last step I took after days of trying everything. I guess I am not used to not needing a washer or a rubber seal but in the case of this piston type valve, the right pressure in the right place seems to work.

    1 reply

    Reply 1 year ago

    Glad to hear you were able to solve it and thanks for the additional technique.

    I noticed there is a flat brass coupling the gray plastic to the free-spinning chrome female nut. In your case, it seems that was enough to create a good seal.


    Question 1 year ago on Step 3

    Hi. It was great to find your post. Did you hand tighten after you installed the brash washer or tighten further with a wrench? I installed a C200 yesterday and was having some leaking at the same spot. I tightened very tight with a wrench. It stopped the leaking but I'm now worried that I perhaps over-tightened, which will cause an issue in the future. Thoughts?