GMC 1993 Jimmy Owner's Manual page 247

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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2.
Then keep turning the pressure cap, but now push down as you turn it.
Remove the pressure cap.
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