With that much fluid, can't you tell where it originates from?

Sounds like a seal is shrinking when sitting.

DAVE,  I HAVE INSPECTED IT AND THAT IS WHY I CHANGE THE PAN GASKETS. I EVEN SUSPECTED THE DRAIN PLUG, EVERYTHING HAS BEEN REPACED WITH NEW MORE THAN ONCE AS FAR AS SEALS GO.

Are there cables that enter on the sides? Have those seals been replaced?

from what i can see and from the origin of the fluid on the ground,  it appears the leak generates from the right side(passenger) of the tranny.

thanks Dave, however, that has already been done.

Here's the most likely culprit....the o-ring around the pump. Doesn't matter if it's been replaced or not, unless it has been replaced with a specific o-ring. I use one from a 700 pump, as they are a bit thicker, and seal mooch better...never had one leak with this set-up.
Here's what's happening...after sitting, the fluid in the torque converter drains back into the pan, raising the fluid level in the pan. When the level gets up above the lower edge of the pump, it will seep out around the pump o-ring, and it may appear that the pan is leaking.
The only solution for this particular issue is to replace the pump o-ring with one that is thicker, and it just happens that a 700 o-ring fits the bill. (Of course...some Bubbas seem to think that two complete tubes of sillycone around the pump will fix it...)
Now, for a dis-claimer.... This will only be the problem if you are 100% absotutely sure it is not leaking anywhere else, and it ONLY leaks after sitting for a few days.
For some reason, the 400 trans is really bad about this.
hth


We posted at the same time, so I didn't see the newest info! The cooler lines MAY be leaking, but you're correct in thinking they would also leak while running...not while just sitting.
If the pump is leaking, then with the engine running, it will "sling" the fluid over towards the pass side, and make it appear to be leaking there...Adams' Apple2008-05-14 10:08:08

I had my gaskets and O rings replaced recently on my '76 at Cottman's Transmissions. I was told the gasket around the shift linkage going into the tranny and the cable O rings are known to leak on these transmissions. So far I have not seen a leak but before I had them replaced I had the same issue.

Bob, unfortunately, the o-ring fits around the outside of the pump. No way to get to it without removing the trans, and the pump...
Also, if the trans has had a "shift kit" installed at any time, or has otherwise had other work done to it, the pressure regulator valve, and/or the pump may have been "modified", which leads to increased converter drain-back, which in turn increases the chance of the pump leaking after sitting for a few days, or in some cases, overnight!

Aaron, the 400 doesn't have a cable going into it like the 350 or 700 does. The shifter shaft seal will leak, tho, same as any of them.Adams' Apple2008-05-15 10:07:06