1990 Buick Century Cooling system air purge

Your Century should have a relief valve on the radiator..but if not..

Remove the radiator cap while COLD, top it off with correct coolant, start the engine, add coolant as necessary until all air bubbles stop escaping. Raising front of vehicle during this procedure will speed up the process. If radiator is already full, just open radiator cap then start vehicle and let it run for 15 mins or so.