Introduction: Fixing Leaking Toto Washlet S300 / S350 Junction Valve
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 both 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.