Did the previous work done include bushing replacement
? Or did the guy that did the first job just do the seal?? If he gouged the bushing bad enough, he could have caused enough clearance to cause a leak under heavy load. The pump/converter bushing can have significant pressure behind it, and if the oil has somewhere to go, it will. That seal will only hold so much pressure and volume.
EDIT: I found this on ALLDATA, and it may relate -
Originally Posted by ALLDATA
Some THM 400/475 models may experience a fluid leak coming from the converter or oil pump area. This could be caused by the front pump bushing walking forward, resulting in fluid being forced past the pump oil seal. A new bushing was introduced that has an increased hardness specification which results in a higher push-out load.
DATE OF PRODUCTION CHANGE: (Figure No. 1)
Beginning August 1, 1988 (Julian Date 213) all 1989 THM 400/475 transmissions were built with the new pump bushing.