With cold engine open radiator cap and add coolant there. The way the overflow is supposed to work is when engine heats up coolant expands and pressure builds, pressure is good it raises the boiling temp, When the pressure exceeds the radiator cap it releases some coolant into the expansion/overflow tank. This might happen on a long grade. When your engine cools back down the coolant shrinks and the pressure drop inside the sealed cooling system, creating a vacuum. The radiator cap is supposed to allow the coolant in the expansion tank to be drawn back into the radiator. If it continues losing coolant it will either be a bad cap or a small leak somewhere not allowing the vacuum created by cooling down to draw coolant back into the radiator.
You might want to call Freightliner with you VIN and ask if your 1998 has an engine protection system that will shut it down when low on coolant. I have driven newer ones that will shut off if the radiator is down 1/2 gallon or so. My solution if that happens is to rough up the ride by abrupt braking or swing side to side enough to slosh the coolant in the radiator around and get the coolant sensor wet again, the light would go off and you could get to where you could add coolant. The early clue is the light may come on momentarily on long sweeping curves, like an entrance or exit ramp or even when making a turn from a stop,when the coolant is sloshed to one side or the other exposing the sensor. It really sucks when your tractor trailer shuts off when making a left in traffic. They will generally start right back up so you can get out of the intersection.
Bill
