I have a 1995 GMC diesel 3500 box truck. It is losing water at a rapid rate which has gotten to the point of losing one gallon in under 100 miles. Two mechanics have put the system under pressure and can find no leaks. The oil and transmission fluid are both clean. Any ideas where the water is going. I quite adding radiator fluid when the leak became near this amount. Both shops say nothing is wrong, the second being a diesel mechanic. Thank you, David