1994 GMC Suburban Owner's Manual - Page 220

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 radiator hose, is no longer hot. P : . Turn the pressure cap slowly to the left until it first stops. (Don't press down while turning the pressure cap.) If you hear a hiss, wait for that to stop. A hiss means there is still some pressure left. P F B 2. Then keep turning the pressure cap, but now push down as you turn it. Remove the pressure cap. 4 I 5 - 16