The bottom of the bell-housing.

Old seals can shrink. When the fluid warms them up, they tend to swell again and seal well. Many stop-leak ATF additives are sold, which basically are solvents that swell the seals. Typically a last-resort and not recommended by transmission experts.

Your seal is likely shrinking as it dries out from non-use as the seal isn't immersed in fluid when the engine is off. Once the engine is running and circulating warm fluid, it tends to swell the seal again, and the residual fluid will keep the seal swollen and sealing for some time after use.

My spare transmissions are kept over-filled (to the top) to avoid this. I have a friend with transmission shops and really old cars, this is how he stores his cars in the winter (overfilled). He then drains the fluid back to normal levels before starting (knows how much extra is in there).

__________________

Gone to the dark side

- Jeff