Chevrolet 2003 S10 Pickup Owner's Manual page 300

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 counterclockwise 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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