Old Gallion grader Transmission leak problem

New to the forum- all input appreciated. We just bought a old Gallion grader with a 2420 transmission and c-273 torque converter in it. The output shaft out of the transmission was leaking oil badly. When took it apart the flange was not very tight and the seal was damaged badly. Replaced the seal and made sure sealing surface on flange was good. When we got it running again it starting leaking again after a very short time. It appears that there is pressure pushing the oil out of the seal, the oil looks like it has air in it coming out around the seal. How much pressure should there be in the sump of the transmission? Any ideas of what may be going on here? Thanks