I have a TH400 that gushes oil from top of fill tube. It isn't running hot, in fact it will do it when trannie is cold. Fluid isn't over full. I checked the plastic vent tube. I can't blow through it but I can suck air through it. I left the trannie hump off and drive it. I can't feel any air coming from the vent. What might be the problem? L.D.