904 leak mystery

I'm having a problem with transmission leak on 1976 scamp 904 .I fixed what I thought was the problem o ring on the filler tube. Add trans fluid to the correct level short trip around the block checked it out through out the day. everything was clean and dry .no leaks.i keep an eye on it for a few days still no leaks.on the third day late afternoon.there was about a half a quart on the drive way.i couldn't tell where it was coming from because there was fluid on bottom bell housing .dripping around pan .has anyone run in to this problem.if so how did you correct this i cleaned it all back up.and refilled transmission to proper level.now it's day two and no leak. I would appreciate any information.what it might be.thank you