A 1948 Chevrolet Fleetline has what is called an "open" cooling system. On an open system, the overflow exits out at the top of the tank. On the more modern radiators, it exits on the neck and, consequently, the cap holds the pressure of the radiator at the flange just below the spot where the overflow exits. When the pressure exceeds the amount of pressure the radiator is designed for and the cap will allow, the cap will release and the pressure and overflow will be allowed to exit the system by pushing on the pressure cap. On an open system, none of this is applicable. The system is designed basically for no pressure at all and any type of cap will have the same results since the overflow exits from the top of the tank and there is not a flange on the neck of the radiator. The only way to control the overflow on a stock 48 Chevrolet Fleetline is to run an overflow tank and the small amount of pressure which causes some small amount of overflow will, as it does on a "closed" system, draw the water back into the radiator as it cools. In most cases this will allow you keep your system full of coolant and function correctly.
Joe
