Fluid Choices

Generally if the trans is leaking when the vehicle sits it's either the shift shaft seal (where the valvebody shaft comes through the case) or the dipstick tube o ring. What happens is the converter drains down while parked and the fluid level rises above these two seals, when the car is running the fluid level is below them. Sometimes the dipstick tube seal area is too deep and the o ring doesn't protrude enough to seal to the case, or it may be bent.

Additionally excessive torque converter hub to front pump bushing clearance can also cause excessive converter drain back as will excessive pump case to gear clearance. Either way though this shouldn't cause a leak if the trans fluid level is correct.