1996 Buick Riviera Owner's Manual - Page 215

How to Add Coolant to the Radiator 1. You can remove the radiator .pressure cap when the cooling system, including the radiator pressure cap.and upper .radiatorhose,-is no longer hot. 2. Then keep turning the pressure cap, but now push down as youturn it. Remove the pressure cap. Turn the pressure cap slowly counterclockwise until it first stops.(Don't press down while turning the pressure cap.) 1f.youheax a hiss, wait for that to stop. A hiss means there is still some pressure left. 1 3. Fill the radiator with the proper mix, up to the base of the filler neck.