If the tank does have a flapper valve, and ii turns out that it is
causing the leak down problem, FluidMaster make a really neat
replacement flapper/seat kit.

Not a lot of money, it includes a stainless steel seat, easily attached
to the old seat with a kind of sticky gasket material that conforms to
and fills any irregularities in the old seat - once pressed into place,
it will last for years.

And ya don't have to take the toilet apart to do the job.

Joe