2005 Ford E-Series Van atf blowing out vent tube

I just did a DIY fluid and filter change to my 5r110w tranny. I dropped pan changed internal filter and also changed external cartridge filter. replaced pan and added new atf. As per instructions from a friend who has been a fleet mechanic for 30 yrs- I removed rear banjo fitting. Started engine for a few seconds, catching atf in a pan. Stopped engine, added another 4 qts atf, started engine again for a few more seconds catching atf in pan. I followed this procedure until I had run 24 qts atf thru the tranny. Buttoned it up, and then added new atf to get it at correct level on dipstik. Took it for a run, stopped and checked fluid level twice, added a little more till fluid level was correct. All seemed ok- shifted well, no slipping. Next morning left on trip to Lake Champlain- made it 30 miles and then smelled atf. Pulled over and found fluid leaking out of vent tube!!! Turned around and came home! What is wrong? Have I damaged my tranny? What do you think I should do next?
Certainly appreciate any insight.
Glenn