A few years back I posted to the group a question about whether or not
anyone had ever had this problem.  It persisted after a complete rebuild.
Without boring you with all the gory details, the gist was that the
breathers (which reside on top of the differential, not on top of the
transmission case) were rusted shut.  A little PB Blaster and some delicate
pliers work and all is well.  This was sleuthed and solved in 10 minutes by
Larry at Rick's Transmission in Ravenna Ohio.  Can't say enough about that
shop!

Just thought I would contribute to the collective brain trust.

bill