Toilet Tank Standpipe

There is a leak we can't find. Replaced float assembly and flapper. It's a slow leak. With the supply cut off at the wall, it leaks down to below the flapper.
How is the standpipe (overflow) removed for replacement? Is there a large nut accessible only when the tank is removed? Could the leak be stopped with Tub & Tile Caulk around the very bottom of the pipe? Help Please!
TKS, Scott
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Scot wrote:

The overflow tube is part of the flush valve assy. Although it does screw into the flush valve casting, it is very unlikely that the threads are leaking. The flush valve can only be removed by taking the tank off the bowl. Then, there is a large nut under the tank. There is a gasket between the flush valve and the upper side of the tank. Rare, but possible that the gasket is leaking and allowing water to run past the big threads and into the bowl.
As an experiment, try sealing the flapper to the seat (caulk?). If still leaks, guess you'll have to pull the tank. Jim
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Jim --     Thanks for the clarification. We'll follow your advice. I believe that will give us the answer. This IS most unusual; never saw it happen before.
-- Scot ----------
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