spewing 727

I got a 727 on a 440 that is spewing its fluid from the vent. I just put a B&M Transpak in it along with a 2800 stall converter. During the test drive I "crop dusted" quite a few people. Got home and checked the fluid level, as expected it was low. I put the car in the air and topped off the fluids. Gently ran the car under no load in the air (stopping tire rotation before changing gears) - all was good until I put it into reverse. At that moment about a quart of fluid spewed out the bellhousing area. I pulled the tranny, thinking it was the front seal and it was not. The converter hub and seal looked great. I believe it came from the vent. Any suggestions as to my next step?