Sorry, that was a misunderstanding of mine. Old-tank supported the info about the front torque tube seal.
Ok, the leak at the rear torque tube is heavy.
But to explain my troubles more clearly here I go:
If I fill the tranny to the full mark at the dipstick and drive the car then I have that heavy leak at the rear of the torque tube.
After each driving and parking the car over the weekend I have an oil spill of about 15"x15" under the torque tubes rear.
Then, after driving and driving again and heavy leaking again, the fluid level in the tranny drops over the time.
Then the amount of leaking seems to go down too.
So I asked myself what's the reason why.
Therefore I measured the ATF volume in the tranny and find out, that I have 1.5 - 2 qts to much ATF in the tranny.
The manual says, that there has to be ca. 10.5 qts oil in the tranny including the converter.
I have ca. 12 - 12.5 qts ATF in the tranny, when filled to the dipsticks FULL mark.
So here is something wrong, isn't it?
So before I make the time consuming work in regards of removing the rear axle and driveshaft to inspect the front driveshaft seal and seal ring bushing at the driveshaft (like old-tanks hint),
I want to figure out the right ATF level in the tranny.
Nevertheless I will drive the car with the correct amount of 10 - 10.5 qts ATF in the tranny (fluid level then at the "I" of the dipstick => see pics above).
So I can check out, If the fluid level could has something to do with the amount of leaking.
So please, if there is someone out there with a 1956 Dynaflow Buick, could you measur the lenght og the upper transmission oil filler tube.
Thanks, Axel