727 Problem

I have a 70 Dart 340 with 727.Had the transmission rebuilt about 3000 miles ago,The problem I'am having is after it sits a day or so I go to back out of garage and when I put in reverse it takes 30 seconds or so for It to move like it has to build pressure then after that its fine other then that the trans works fine.I have heard this is called morning sickness.What causes this and can it be fixed in the car.Its not a big deal but would like to know what my options are Thanks..