1994 GMC Suburban Owner's Manual - Page 226

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. 2. Then keep turning the cap, but now push down as you turn it. Remove the pressure cap. 3. Open the air bleed valve located on the thermostat housing. 4. Fill the coolant surge tank with the propermix, up to the COLD mark. I