There's a small rubber O ring in there they may have been either left out or damaged when installed. You'll have to drop the pan to fix it but not the trans. You might have to remove the valve body to depending on the lever into the trans setup. But you do not have to remove the trans itself.
Yea, I just did the same job on my C4 and I had to drop the pan, if I remember correctly, the outside o ring is more like a dust seal, I seem to remember having to change another seal on the inside, but I kinda forget so don't qoute me on it.