3060 blowing fluid out the vent

Allison Man
I have a 1997 3060 coupled to a 8.3 Cummins that is blowing fluid out the vent on top on transmission. It has a retarder. It seems to do it when I use the retarder. I have checked the air cylinder by disconnecting the air and fluid lines and applying shop air pressure to the air end. It is not leaking by. What else could be causing this foaming? The retarder seems to become ineffective when the transmission gets about 1 to 1 1/2 qts. low. It will lose about 1 1/2 qts. driving from Yuma, AZ to the lower Texas Valley, approx. 1300 miles.
3060 blowing fluid out the vent

What type of selector do you have? Can you check the fluid level by the selector? If it is over filled it will eject the fluid through the vent on the top. Just food for thought. Good luck.