I have a mancesa 1.6 gpf model as well, I placed on new bolts from tank to base. Then placed on new flapper, I feel moisture under the lid base despite the bolts being very tight. I think the issue is the gasket at the base. Does anyone know how to redo the gasket at the base? It seems I have to remove the whole base since the gasket is under the base.

Most of the time, the leak is caused by bolts. Do this: wipe off all water from the toilet bowl. Do not flush. Leave full tank of water stand over night. In the morning, see if there is wetness. If yes, bolts leak. If the bowl is dry in the morning - then flush several times and see if there is sign of wetness now. If yes, than it is the bowl gasket.

Hope that helps

Milo