Buick 1993 LeSabre Owner's Manual page 215

How to Add Coolant to the Radiator
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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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