I can definitely see any roaming mice or rats creating a problem with chewing/flooding. I have not run into this, as we don’t have much for wild rodents here in Vegas. I have seen where others keep a few live traps around just in case of escapees. As for the leaking valves... luckily, I have not run into this at a year in. As it has been said, "they are food, but still part of the family". A flood would be extremely sad.

Anyways, I found this youtube video that pretty much shows the basics of putting the auto/gravity water system together. There are many different configurations and tweaking you can do (I liked adding on off valves for bins not in use, even though I haven't had that scenario for a while); however, this will show you the basics of setting one up if you are still interested.

http://www.youtube.com/watch?v=uh1HUULQjoM

Here is the link/guide I used for my rack build. Lots of pics, solid rack, and shows more watering examples.
http://www.southtexasreptiles.com/st...ead.php?t=3318

The only major difference with the rack was the rails. I too had to double up my rail width in order to hold the tubs more securely.

Good luck, and let me know if you have any questions! Will be happy to help in any way I can.