If it isn't leaking externally, which would be highly visable, then it may be leaking internally at one of the brake applicators and then be flowing into the axle housing, or into one of the planetary housings and then back into the axle housing.
It may be bypassing at the foot valve right back over to the return side and back to reservoir.
I do not have any specific ideas as have never had to work on this model, but it's not hard to check the possibilities I listed as an easy 1st or 2nd step in trouble-shooting.
Good Luck!