tranny leak at the dipstick tube

I put the car back together after the resto and decided to go to the local car show and enter it. did great except for this vibration I mentioned.took a trophy. came home parked it in the garage and got up the next morning and went out to get something and about had a heart attack. There was tranny fluid all over the floor. cleaned up the mess. filled it up and checked it. no leaks. ran it, got it hot no leaks. Put a pan under it and let it set and as it cooled off, saw it starting to drip. It was coming from around the dipstick tube. It is a 727. It seems to only leak when it cools down. I am almost sure I put a new o-ring on there when I put it back in. Is there a vent on the tranny somewhere that is building pressure that is causing it to blow out of the tube. I noticed when I took the tranny out that someone had put silicone around the dipstick hole before. I thought that was odd but maybe not. The tranny did not leak before I did the resto. Any thoughts on this. thanks,Bill