Water

[ClayPigeon]

Create an entity that consists of a water material(the game already has that material, just create a plane with some physical attributes) and use a constant pulse on it(AddPropForce).

EDIT:
Alright, I've made a simple map with the water entity that drains down. It's pretty simple but it's buyable, lets say.
Download the custom story and mess with the propimpulse value to change speeds.
I didn't do the lever part, it's relatively simple and I think you can do it yourself.
http://www.solidfiles.com/d/0e84b7a55c/