I have been experimenting with fillings other than buttercream lately; namely curds, purees, and caramel. I have found, though, that they don't really "fill up" the space between the layers. I pipe a dam and then put in my filling but the filling does go all the way up to the top of the dam. As I stack the torted layers up, I develop a sag in the middle. I don't add any more of the runny fillings for fear they will run over the dam. I hope I am explaining this properly. What can I do to avoid this? Thanks!

make your dam big enough to hold the filling, but reduce the height of the dam. Sounds like it's not so much that your filling level is low, but your dam height is too tall.

Thanks Indy! That worked really well. I guess I was afraid of piping a lower dam for fear of my filling running out. No leakage!