My understanding is that when the coolant heats up, and the radiator cap exceeds releasing pressure, coolant will flow into the overflow. When the system later cools down, it ought to create suction and draw coolant back into the radiator from the overflow. On my 86 though, the coolant always ends up like an inch down or so from the top. I don't think this itself is a big deal, but should it really be able to stay totally full if the system is working right? There is coolant in the overflow, the cap is new and the tubing is in good condition.