This is a common problem, not only on T400's. I've got a Trimatic behind a V8 in one of my Toranas and if I fill it with the correct amount of fluid it shows LOW on the dipstick and the trans slips when cold.

So if I fill it to the FULL mark on the dipstick, it doesn't slip when cold and operates correctly, but leaks when parked for a while.

I have an oil cooler mounted to the radiator instead of using the tank on the bottom of the radiator, so maybe I have a larger cooler than the tank which would explain the trans slipping when cold once the fluid circulates into the cooler.

The leak could be because once I park, the fluid drains from the cooler to the trans and is way above the normal level, possibly then leaking from around the dipstick tube.

So are you running a cooler like me, or the normal tank?