It is not uncommon to have them drip a bit at the collar. The seal in the collar could be cork or rubber which is a later seal.
However if you fill the tranny and drive it a short distance and then it is empty and the diff has more fluid, then your seal has gone. If the fluid in the diff stays the same but you are losing a lot from the collar then I would be concerned about that seal. IMHO that would be the best way to test it as that is how I discovered my seal was gone. As you are doing this you want to make certain that you don't get too much fluid in the rear as it will push out the seals. You should also check that your rear axle breather is working at this time.


1949 Chevy 1/2-Ton
"Sedgewick"
In the Gallery
1989 Caprice