1998 Dodge Dakota Can't find the leak?

I have a leak somewhere on my truck in the brake system. I can't find it and don't have a clue. I was pumping the brakes sitting in the Drive and I can't get the resivor to drop level. However when I drive it the level will go down, I just replaced the rear pistions and this showed up like 2 weeks later do I have a front caliper leaking? The rear looks fine. I just can't seem to get them to leak unless I am moving and applying the brakes. any ideas??????