7.3 BAD coolant leak...passes pressure test
 

I have a 2001 F550 7.3 (and it happens to be my only service truck--very important truck in my world) and it leaks coolant BAD, especially under load. I have not been able to find the leak visually and it loses nothing on pressure tests (it's been tested twice). All radiator and heater hoses have been replaced (they were spongy and on their way out anyway), it has a new radiator cap, and no there is no water in the oil. Anyone got any suggestions? I am all ears...I am out of business without this truck...any help would be appreciated.