Coolant level dropping fast

[Double Stack]

Just bangin my head against the wall over here...

My problem is as fallows, my coolant level is always dropping. I'd say I need to add about a half a gallon of water every other time I drive it. Or say every 40 to 50 miles. I've driven it hard, and then checked for leaks. I've had somebody watch me drive away as I get on it, as i've read on here that it could be coming from a freeze plug under load as I get on it. I've pressure tested the coolant system and there were no leaks. It also held 12 1/2 lbs of pressure for like five minutes. There's no coolant/water in the oil. I also dropped a surge tank in the truck to see if it was pushing water out it overflow and not a drop was collected.

Talked to a guy I know that's pretty smart, and the best he could come up with is if i'm running mostly or all water in my coolant system it could be cooking off fast. I told him it seemed to doing it back when i was filling it up with a good mix of coolant too, but i've been running straight water for awhile now.

Anybody feel like being a GENIUS!?!