Buick 2004 Regal Owner's Manual page 244

1. The radiator pressure cap can be removed when
the cooling system, including the radiator pressure
cap and upper radiator hose, is no longer hot.
If a hiss is heard, wait for that to stop. A hiss means
there is still some pressure left.
5-36
2. Then keep turning the pressure cap, but now push
down as you turn it. Remove the pressure cap.
CAUTION:
{
You can be burned if you spill coolant on hot
engine parts. Coolant contains ethylene glycol
and it will burn if the engine parts are hot
enough. Do not spill coolant on a hot engine.
3. Remove the 3800 Series II V6 engine cover shield
to access the bleed valve.