Cadillac 2003 Seville Owner's Manual page 285

1. Park the vehicle on a level surface. You can
remove the coolant surge tank pressure cap
when the cooling system, including the coolant
surge tank pressure cap and upper radiator hose, is
no longer hot. Turn the pressure cap slowly
counterclockwise (left) about one-quarter turn and
then stop.
If you hear a hiss, wait for that to stop. A hiss
means there is still some pressure left.
remove it.
2. Then keep turning the pressure cap slowly, and
3. Then fill the coolant surge tank with the proper
mixture, to or above the fill mark at the forward
edge of the surge tank.
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