I am the original owner of a 1999 Tahoe Sport 4 wheel drive with the Z-71 package. It has been an excellent vehicle. There has been a problem with it that started about 10 years. The water level in the radiator runs about 6 inches down from the cap which means the water overflow tank is always empty. If I fill both the tank and the radiator up it just goes back to the radiator going down to 6 inches below the cap after it is driven awhile. I did buy a new radiator cap at one point but it didn't help. The antifreeze does not drip out on the floor. The oil has zero residue in it and there is no residue on the oil cap. There is no smoke coming out of the exhaust and the vehicle does not over heat even when driving long distance with the outside temps being in the high 90's. I have let it go this long because no mechanic so far has been able to tell me what needs to be replaced. The engine is the original 350. Is there anyone out there with a viable idea?