Cadillac 1999 DeVille Owner's Manual page 254

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1. You can remove the coolant surge tank pressure cap 2. Then keep turning the cap, but now push down as
when the cooling system, including the coolant surge you turn it. Remove the pressure cap.
tank pressure cap and upper radiator hose, is no
longer hot. Turn the pressure cap slowly
counterclockwise (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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