Had my 904 reconditioned two years ago but now it started leaking all over the place, not happy! Looks like it’s the torque converter seal, at least that’s where it seems to be coming from after letting the car sit overnight. The seal itself isn’t all that expensive but how big of a job is it to replace the damn thing? I won’t be doing this myself but not sure if I can trust a regular shop with it either.
Why did it fail in the first place, could this be an installation error when they rebuilt the transmission? I get a lot of vibration at certain speeds which according to a specialist workshop doesn’t come from the cardan shaft. Could be the engine (also rebuilt) or transmission which I obviously wasn’t keen on taking apart again just yet.

Anyways the leak really needs fixing now.

depending on where the leak is, it could make it look like the converter.

first thing is to see if you can track the leak.
typical leak points are the selector shaft seal. lots of shops don't bother to replace the seal when apart for some reason.
feel up around it and see if it is wet there first.
It could drip down to the front and make it appear to be a leak infront.
The other choice is the dipstick tube seal. since it is only an o-ring.

If both those places are dry, I think there was a vent that could puke stuff out if the trans were over filled and foaming.

I am sure others will have ideas shortly.

I assume the leak is dripping off the bottom of the bellhousing. You need to determine if the leak is coming from inside the bell, or from outside the bell.

Remove the tin cover on the front of the trans and see if there is trans fluid inside the housing. It the leak is inside the housing, it is likely the torque converter, pump seal, or the converter itself. A leak there will require dropping the trans out of the car to fix the leak. An out of balance motor or converter that is causing a vibration can cause the converter seal to fail.

If the inside the housing is dry, the leak is likely not from the converter related parts. Slow outside leaks tend to travel along the transmission pan, then run down the bell and drip off the bell because it is the lowest point. As stated above, the three most common outside leaks are the pan gasket, the gear selector/kick down lever, or the dipstick tube "O" ring.
The selector seal is on the left side (as your sitting in the car) of the trans, with a small mirror, you may be able to see the fluid running from the bottom of the levers towards the trans pan. That seal may be able to be done in the car, but it is a real pain to do.

The dipstick tube is on the right side of the trans, and again, you should be able to see a fluid trail from the dipstick to the trans pan. The dipstick tube "O" ring can be changed from under the car, but the fluid may need to be drained down slightly before you pull the dipstick tube out of the trans.

The trans pan gasket usually shows signs of having been over tightened and the gasket will likely be squished out where the leak is, but not always.

Just above the rear edge of the trans pan is a round shaft that comes through the case from inside the trans. It is a pivot shaft for something inside the trans. I have seen several transmissions leak around the outside of this shaft. I have also seen the tail stock gasket leak.

As you see ts important to find where the leak is coming from before it can be fixed.

As for the vibration, you will want to check the u-joints in the driveshaft to be sure they are both good, and you want to be sure the motor and trans mounts are not broken. Gene

If it leaks after setting over night it may be the dip stick tube seal leaking when the anti drain back valve for the converter in the valve body doesn't seal up properly, not uncommon on theses trannys

No such thing as an anti-drain back valve in a TF.

Drain back is perfectly normal.