I've got a gallon bottle of Tmax developer that's %75 full.
I've also got an unopened 16 oz bottle of the same
and an unopened bottle of Tmax RS developer.
All is at least several years old.
Could anyone hazard a guess as to how viable they are ?
Throw away the open one, it is bad. Tmax goes bad after a while after opening, like most developers. The unopened stuff has a very long shelf life. Those bottle made in the last several yrs actually have expiration dates printed on them.
A guess is just a guess but unless they've turned dark brown I'd give them a try. Kodak b/w chemistry is pretty famous for a long shelf life. The only proof will be in the development.
Open the small package with the tmax rs developer and empty it into the big bottle.Shake.Put todays date on it.Develop tmax 100 and tmax 400 at 1+9 dilution.Be impressed.When it's gone get more and repeat.