Hi nu-wax seal & installed new fluidmaster 404lg. All works fine except next day, no water in tank. (leak guard works) why is flush valve not working. All is new. I have done this several times over the years with no problems. Flush valve is not wavy. I also cleaned out inside of tank completely.

Thank you

Do a quick dye test... betting the flapper isn't sealing right... even if it isn't "wavy".

Drop about 20 drops of dark food coloring into the toilet tank full of water and let it sit for a couple hours. If colored water shows up in the toilet bowl then the flapper isn't sealing and should probably be replaced.

On occasion, some new flappers simply won't seal due to the way they were packaged or some other silly reason. Sometimes, if the flapper sits for a couple days it can resolve, but most of the time, replacing with a new flapper resolves the issue fastest!

Back to you...

Mark