Do both front tires deflate when the disconnects are disconnected?
A way to isolate your prblem is to inflate all tires to 40 or 50 pounds (this is from the book). Disconnect all valves and wait. This will tell you what tire(s) it is in. It does sound like it is in both fronts but that is a hint from the book.
From the sounds of your problem it is both the system and the tires. When the tires are disconnected the system can loose air but it shouldn't. The guage will read zero if there is a leak and the gauge should be ok at this point.
You can also try searching here:
http://www.amghummer.com/humdefault.htm
and here:
http://www.humvee.net/