I have not put the car in the air yet to investigate where it might be coming from... it's a TH400 and interestingly, there are no drip marks on the garage floor :confused:

Any ideas?

Blowing out the vent?

You might check the tailshaft seal where the driveshaft enters the transmission . I just changed mine this past weekend and it had the same signs as you mentioned. Good luck.

My pickup had the same issue. Replaced the seal and it got better, but the problem was not fixed. I replaced the output shaft and the problem went away. The old one did not look grooved or worn too bad, but we're talking about very tight tolerances.

Tranny pan bolts loosening up.
Drain plug if you have one, seeping

I'm not sure if the dipstick tube fits in in a similar way as on the TH350, but my TH350 leaked at the dipstick tube pretty bad and it looked like it was coming from everywhere else. The dipstick tube, if it's like a TH350, has an o-ring on it where it goes into the transmission case. Replace this o-ring and add a small bead of sealer around the tube where it joins the case and you'll probably clear up all or a good portion of your leak(s).

Make sure your cooler lines are very tight, lotsa messes come from them when pressurized, and the fluid goes EVERYWHERE!!!

Its late. I have to say it. Rustproofing!?

Thanks everyone... I will check all these possible sources.
Question: What is the best way to build pressure inside the tranny? Do I have to drive it? I would prefer not to drive it until I find the leak.

Kev, I guess "rustproofing" would be the upside here! LOL!