I had the problem of coolant not being sucked back into the radiator on a 1993 Oldsmobile 88 that I once owned. The problem was the hose between the radiator and the reservoir. The hose would allow the coolant from the radiator to flow into the reservoir, but would collapse when the radiator would try to suck it back in as the car cooled. THe repair was simple and inexpensive–$2 for a piece of new hose and 10 minutes to install.