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.

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.

THAT'S IT!

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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.

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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.