Fixing Leaking Toto Washlet S300 / S350 Junction Valve


Introduction: 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 conical rubber gasket, flat seals, o-rings, sealant tape, etc. to no avail. The working solution was not my first intuition, but is simple.

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

If the tank connection end is perfectly flat, then you might be able to follow the official Toto instructions as they are and have the fiber washer seal correctly.

That did not work in my case, even after purchasing a new toilet tank float intake and another valve set thinking my original THU9339 Junction Valve was defective or the THU9340 Fiber Washers were bad.

Step 2: Inspect the Fill Tank Connection End

If the fill connection to the tank is conical on the inside like many toilet tank floats, then that means you may need to install an extra gasket to avoid leaks.

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

Unlike the official Toto instructions of Junction Valve > Fiber Washer > Tank Inlet, add an additional FLAT brass washer on top of the fiber washer.

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.

The new configuration should be Junction Valve > Fiber Washer > Brass Washer > Tank Inlet

Now attach the Junction Valve to the toilet tank as normal. I did not use any teflon tape in any parts of the assembly. I've read that is only to be used in non-compression-type fittings.




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    6 Discussions

    The installation guide is simplified enough to let most owners ignore the installation order which is critical. I had the same problem on my C100 washlet and had installed it several times. Finally I found that you have to install the connection of *wall* water supply hose to the junction valve first (the bottom), the hose to the washlet(middle), then the tank short inlet to the junction valve(top of the 3-point). An advantage of installing in this way is that you can keep watching if the top part is installed vertically straight. You may need several times to get it right, just be patient.

    So glad to find this thread! At my wits end over here with the Toto C100. Same leaking problem. I've tried the flat washer--that made the leak worse for me. Tried no washer--that was better at the top of the tank inlet. But now, it looks like the water is leaking between the top nut and the gray plastic diverter piece on the Toto part. Starting to wonder if the part itself is damaged--there seems to be a lot of give and sliding on the part. Any suggestions before I send away for a replacement diverter piece?

    1 more answer

    The leak you describe was the main problem I was having. I'd check to make sure the plastic threaded inlet tube is FLAT on the bottom and not damaged from messing with it. The fibre washer is meant to expand and seal, though needs a flat surface putting pressure on it. For me, that's where the 3rd-party flat brass washer came into play. I hope this helps and you are able to fix it. So frustrating! :)

    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?

    1 more answer

    I hand tightened as much as I could go and left it at that. I may have use a pipe wrench for the last 30º or so.

    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.