Chevrolet 1994 Blazer Owner's Manual page 217

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. 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.
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