[G.M.]

I sounds like someone put a pressure tank in the over flow. I never used or saw need for these. I believe there is a pressure cap on the top of this tank. When this type is used the cap on the radiator is a plain cap, not a pressure cap. The tank could also be just an over flow tank. The over flow tube go's into the bottom of this type tank or bottle and an over flow tube comes off the top of this tank to release pressure or excessive coolant. If the engine gets hot or compression pushes some water into the tank it stores it and as the temperature drops through 155 degrees when cooling down the jiggle valve on the pressure caps opens from a vacume in the system and sucks the water from the tank back into the radiator. The purpose of the pressure cap is to hold the water in the system as coolant is forced through the radiator tubes plus the pressure cap is also a safty valve. The pressure rateing of the cap determines the safty factor. A 4lb cap is all that is required on a good working cooling system on these cars. The temperature should never get much over 200 degrees, at 200 there is less then 1 1/2 lbs pressure. Driveing the radiator is cooled by the air forced through the radiator from forward motion and cooling is not a problem. What has to be avoided is idleing or heavy stop and go traffic on very hot days. The fans in these cars don't provide enough air for these conditions. G.M.